Rashi (Bereishis 37:25) quotes the Midrash explaining Arab caravans ordinarily carried foul smelling cargo like tar and petroleum, yet Hashem ensured the caravan Yosef was sold to would carry pleasant spices instead. That being the case, how could the nice aroma really help Yosef get through the frightening journey and excruciating betrayal of his brothers?
Rashi doesn't say anything about the spices being helpful for getting over emotional trauma.
וגמליהם נושאים וגו' - למה פרסם הכתוב את משאם להודיע מתן שכרן של צדיקים שאין דרכן של ערביים לשאת אלא נפט ועטרן שריחן רע ולזה נזדמנו בשמים שלא יוזק מריח רע
He explains that they brought spices so that bad smell of their usual wares wouldn't bother him.
A good answer is provided by Rav Chaim Shmulevitz zt"l in his Sichos Mussar, Maamar 16. He describes how Yosef had experienced a great deal of trauma - he was the 'ben zekunim' (the son of old age) of his father, who was now being sold into slavery, heading to Egypt which was entrenched in tumah (impurity). It was pretty much impossible to escape from a place like Egypt, and it all happened so suddenly. As such, Yosef was in a state of mind where he could well have completely given up all hope.
However, by having the sweet smelling spices it was sign from Hashem that Yosef had not been forgotten. Hashem was still very much there 'holding his hand'. Rav Shmulevitz writes:
בשעה קשה זו נרמז לו מן השמים שאין הדבר כן, ד' עמו ועדיין תחת השגחתו יתברך, ואין לו לאבד את בטחונו בהשי"ת. הקב"ה מזמין לו ריח טוב שלא כדרך הטבע , להורות לו שאין "אבוד" ו"נשכח", אלא אדרבה הקב"ה מוליכו יד ביד והוא וירד עמו מצרימה, כדרך שאמר הקב"ה ליעקב: "אנכי ארד עמך מצרימה ואנכי אעלך גם עלה" (בראשית מו:ד) ריח בשמים זה הרי אוה קרן אורה בחשך שבו הוא נתון, ועל ידו נפתח ליוסף פתח תקוה לחזור ולהתרומם משפל מצבו, כענין שאמר יונה הנביא (יונה ב:ה): "ואני אמרתי נגרשתי מנגד עיניך [העלמת עין ממני] אך אוסיף להביט אל היכל קדשך". ופרש"י "כשראיתי שקיימתני כל הימים, ידעתי כי אוסיף להביט אל היכל קדשך"
At this hard time it was hinted to him from Heaven that the matter was not so (i.e. so difficult and terrible), Hashem was with him and he was still under His blessed Providence, and he mustn't lose faith in Hashem. G-d prepared for a him a nice smell that was not the natural way to teach him that he was not "lost" and "forgotten", and rather quite the opposite, Hashem was leading him hand in hand, and descending with him to Egypt, just like what Hashem told Yaakov -"I will go down with you to Egypt, and I will also bring you back up" (Bereishis 46:4). The smell of spices, was a sign. It was a ray light in the darkness in which it was placed. And through this, it opened to Yosef a sense of hope that he would rise again from the depths of his situation. Like it says by Yonah HaNavi (Yonah 2:5) "I thought I was driven away Out of Your sight (i.e. You turned a blind eye to me), Would I ever gaze again Upon Your holy Temple?" - And Rashi explains, "I saw that You have kept me alive all these days. I know that I will continue to gaze upon Your Holy Temple." (My translation and emphasis)
Thus, according to Rav Shmulevitz, the sweet smelling spices served to show Yosef that Hashem was with him, and that all the suffering he was to endure still came from a place of love.
"The Torah records that the caravan that transported Yosef to slavery in Egypt was carrying fragrant spices such as balsam and lotus. Chazal and Rashi wonder why the Torah records this seemingly-trivial detail, which appears entirely irrelevant to the story. Why does Hashem feel it is important for us to know the cargo contents of the caravan?
Rashi (to Bereishit 37:25), following Chazal (citing Bereishit Rabbah 84:17), explains that normally Arab caravans carried foul-smelling items such as resin and tar. Hashem arranged that the caravan transporting Yosef would feature fragrant spices so that Yosef need not suffer from the malodorous wares.
This teaching is quite puzzling. Yosef is being transported to Egypt to live a miserable existence as a slave. Yosef’s privileged life as Jacob’s favorite son was transformed instantly to a wretched reality. How would Hashem’s arranging for the caravan to carry sweet smells help Yosef in any significant manner? Odoriferous cargo would have appeared to constitute the least of Yosef’s newly-encountered problems. The situation would seem analogous to someone who, God forbid, was kidnapped by ISIS and Hashem arranged for the vehicle transporting the victim to be pleasant-smelling. What benefit does the victim in such horrific circumstances have from the pleasant smell?
One answer is that Yosef was a highly intelligent person, as Onkelos and Rashi (to Bereishit 37:3) point out. He therefore realized that it was unusual for an Ishmaelite caravan to be carrying sweet-smelling spices. Add in the fact that caravans tend to be terribly malodorous because they are filled with sweaty men and animals traveling through a scorching desert, and having a good smell in such circumstances becomes not just rare but almost unheard of.
Yosef realized that this rare occurrence must have been a subtle message from Hashem that He is with him and that He orchestrated his sale to Egypt for a purpose. This message Yosef eventually articulated confidently to his brothers. Yosef did not need a divine revelation to ascertain this information; he needed only a sense of logic and sensitivity to his surroundings to become conscious of Hashem’s involvement. The ongoing and eventually dramatic turn of events during Yosef’s time in Mitzrayim (Egypt) encouraged the same train of thought and led Yosef to detect and identify Hashem’s continued support and intervention."
I infer from his chiddush that had Yosef gone on a regular caravan, with the foul-smelling "works", he would have felt total despair. This uncharacteristically sweet-smelling caravan was a sign of continued hashgachah from Hashem.
I understand the question as follows:
Why would Yosef honestly care what the camels were carrying? Let us say you are being kidnapped and the kidnapper is wearing some nice aftershave, would that really cheer you up?
The answer would appear to be that this was a sign to Yosef that God was with him in his travails.
There is, perhaps, a theme in the thought of Chazal that even though a righteous person must undergo a specific hardship or trial, it can be made as pleasant as possible under the circumstances. Thus, Sotah 12a reports about the pitch made outside of Moshe's ark (compared with Noach's ark):
ותחמרה בחמר ובזפת תנא חמר מבפנים וזפת מבחוץ כדי שלא יריח אותו צדיק ריח רע
The verse continues: “And daubed it with bitumen and with pitch” (Exodus 2:3). A Sage teaches: She daubed bitumen on the interior and pitch on the exterior, so that righteous person, i.e., Moses, would not smell a foul odor, such as that of pitch.
So too here for Yosef's conveyance to Egypt, in Bereishit Rabba 84:17, which is Rashi's source:
אָמַר רַבִּי אַבָּא בַּר כַּהֲנָא וַהֲלֹא אֵין דַּרְכָּן שֶׁל יִשְׁמְעֵאלִים לִהְיוֹת טְעוּנִים אֶלָּא עוֹרוֹת וְעִטְּרָן, אֶלָּא רְאֵה מַה זִּמֵּן הַקָּדוֹשׁ בָּרוּךְ הוּא לְאוֹתוֹ צַדִּיק בְּאוֹתָהּ שָׁעָה, שַׂקִּים מְלֵאִים בְּשָׂמִים כְּדֵי שֶׁתְּהֵא הָרוּחַ מְנַשֶּׁבֶת בָּהֶם מִפְּנֵי רֵיחָן שֶׁל עַרְבִיִּים.
that besides the typical smell that he would otherwise suffer (which is tar, same as what Moshe avoided), there was also the typical odor of these merchants, perhaps because of their dealing in such odiferous substances. The wind blowing on these sweet spices helped mitigate this.
The midrash doesn't use the word נזק, but that is Rashi's expansion of the idea, where Rashi saw the original source, and so I would not read to much into this word choice. In context, the point is avoidance of additional negative smells, and the "harm" avoided is the unpleasant odor, rather than any physical bodily long-lasting injury. Hezek re'iyah is harm, but not physical bodily long-lasting injury.