Why kiss Tzitzith?

Question

Why do we kiss Tzitzith when making the Birchath HaMitzvah, reciting Shema', and after Baruch SheAmar (at least acc. to Minhag Ashkenaz)?

An acceptable answer (for my purposes) would either

a. show that there are other items used for a Mitzvah, other than items with specific holiness, such as Tefillin, Sifrei Torah, etc., which we kiss while performing the Mitzvah or while preparing to perform it, and that this sort of thing is very common across various types of Divrei Mitzvah

or

b. show that this is unique to Tzitzith but that there are good reasons for kissing them at specific times (as in those other answers).

While I'm slightly inclined to think that b. is the answer, as I cannot think of other Divrei Mitzvah that we kiss, there is a nagging feeling that there is something either about doing a Mitzvah (and then we have to explore why we don't kiss other Divrei Mitzvah) or something special about Tzitzith that can be pinpointed, because not only do we kiss them at various parts of Tefillah (and Tzitzith are not only for Tefillah), but we kiss them when we don them (ie., we kiss them when we perform the Mitzvah).

(Speculation: Could it be that kissing them is either part of the Mitzvah or a Hiddur Mitzvah, akin to shaking a Lulav or wrapping Tefillin around the fingers?)

Answer 1

The Kitzur Shulchan Aruch Orach Chayim 11:23 basically answers your question in Hilchos Mezuza:

אָמְרוּ רַבּוֹתֵינוּ זִכְרוֹנָם לִבְרָכָה, כֹּל מִי שֶׁיֵּשׁ לוֹ תְּפִלִּין בְּרֹאשׁוֹ וּבִזְרוֹעוֹ וְצִיצִית בְּבִגְדוֹ וּמְזוּזָה בְּפִתְחוֹ, מֻחְזָק הוּא שֶׁלֹּא יֶחֱטָא, שֶׁהֲרֵי יֵשׁ לוֹ מַזְכִּירִין רַבִּים וְהֵן הֵן הַמַּלְאָכִים שֶׁמַּצִּילִין אוֹתוֹ מִלַּחֲטֹא, שֶׁנֶּאֱמַר, חוֹנֶה מַלְאַךְ ה' סָבִיב לִירֵאָיו וַיְחַלְּצֵם.‏

Our Sages of blessed memory say: He who has tefillin on his head and on his arm, and wears tzitzis on his garment, and has a mezuzah on his doorpost, is sure not to sin, because he has many reminders and these are the guardian angels who save him from sin. (Menachot 43b)

It is also interesting that each of these mitzvos have a halacha about kissing them.

Tefillin (10:20 and also 10:17):

מִנְהַג חֲכָמִים לְנַשֵּׁק אֶת הַתְּפִלִּין בִּשְׁעַת הַנָּחָתָן וּבִשְׁעַת חֲלִיצָתָן.‏

It is a custom ordained by our Sages to kiss the tefillin when putting them on and taking them off.

Mezuzah (11:24):

וּלְפִי שֶׁהַמְּזוּזָה הִיא לְהַזְכִּיר יִחוּד שְׁמוֹ יִתְבָּרַךְ, לָכֵן כְּשֶׁיֵּצֵא מִפֶּתַח בֵּיתוֹ וְכֵן כְּשֶׁנִּכְנָס יְנַשֵּׁק אוֹתָהּ.‏

Since the mezuzah serves as a reminder of the Oneness of Hashem's Name, therefore, when leaving the house and when entering you should kiss the mezuzah.

Tzitzis (17:7):

וּכְשֶׁאוֹמֵר וּרְאִיתֶם אוֹתוֹ, נוֹתְנָם עַל הָעֵינַיִם, וּמִסְתַּכֵּל בָּהֶם וְנוֹשְׁקָן. וְנוֹהֲגִין שֶׁבְּכָל פַּעַם שֶׁאוֹמֵר תֵּבַת צִיצִית, נוֹשְׁקָן, וְאוֹחֲזָן עַד וְנֶחְמָדִים לָעַד, שֶׁאָז נוֹשְׁקָן וּמַנִּיחָן מִיָּדָיו.‏

When saying Urisem oso, (and you will look upon it) touch them to your eyes, look upon them and kiss them. It is customary to kiss them each time the word tzitzis is said. They should be held until the words, Venechmodim load are said; after which they should be kissed and released.