I heard the following explanation from Rav Yonason Sacks regarding the difference in obligation between the two mitzvos that we wear - tzitzis and tefillin.
The mitzvah of tefillin carries an absolute chiyuv (obligation), while the mitzvah of tzitzis only applies if you opt in. You only become obligated in tzitzis if you choose to wear a garment with four corners; without that choice, you have no obligation.
He explained the reason for the difference as follows:
The Gemara in a number of places (including Berachos 11a) describes tefillin as a p’eir - an adornment. The Gemara says that this is true to the point that one doesn’t wear tefillin during the first day of aveilus (mourning). Even though once his relative’s body is buried the mourner is no longer an onein and is obligated in all other mitzvos, he is not allowed to wear tefillin, as doing so is considered a contradiction to his state of mourning. The way Rav Sacks said it is that it would be like coming to a funeral dressed in extravagant, royal robes; it’s totally inappropriate given the nature of the situation.
As a symbol of royalty, tefillin represent that that the wearer is a ben melech - a child of the King.
When it comes to tzitzis, on the other hand, Tosfos on Menachos 43b (ד״ה חותם של טיט) say that tzitzis symbolizes avdus (servitude), like a garment or uniform that a slave wears to represent that he is a servant of his master.
Rav Sacks pointed out based on this distinction that, while maybe counterintuitive, we see that Hashem demands that we see ourselves as children and requests that we see ourselves as servants. Apparently to be an eved just because the Torah demands it and we have to is not the kind of service that Hashem is interested in. Hashem wants us to opt in.
