There is a beautiful take on Tefillin at http://www.ravkooktorah.org/KI-TAVO-69.htm that i would like to share.
It is known that one who is mourning is obligated in all Mitzvot except Tefillin. Why? The Jewish Sages explained that Tefillin are called “Pe’er”- the Hebrew word for splendour. After the prophet Ezekiel’s wife passed away he was told not to observe the usual mourning customs. Therefore, he was told, “Bind your splendour upon yourself”- meaning that he continued to lay Tefillin, which is not the usual custom for a mourner.
Also on the ninth of Av, the day on which we mourn the major communal tragedies that befell our people over the centuries, we may not wear Tefillin in the morning hours, when the mourning is at its most intense. Why?
Rav Kook explains why through elaborating on the meaning of “Pe’er”. Splendour is impresses those who view it. A Mitzvah done in a beautiful way we combine the inner emotions and outer beauty. The Mitzvah of Tefillin has the potential to be splendorous and thereby to impact others.
Rabbi Eliezer explained that in Deuteronomy it is written that “All the nations of the world will see that G-d’s Name is called upon you and they will be in awe of you.” (28:10). He comments in Berachot 6a that this verse refers to the head Tefillin. Tefillin contain passages that include G-d’s Name from the Bible. Therefore they can potentially inspire others when they see how “G-d’s Name is called upon you.”
This is the reason why mourners cannot wear Tefillin.
In order to influence others, one must be joyful and full of strength. This inner joy can then spill out and be utilized to influence others and show them the light.
As King Shlomo said in Kohelet 3:1, “A time to cry and a time to laugh; a time to mourn and a time to dance”. There are times in our life cycle when it is correct to mourn and to cry. At such times, our hearts are heavy and this is not the time to reach out to others. This is the time to draw within oneself. No good will come of trying to influence others from a place of sorrow and grief. This will only spread sadness.
Therefore, during times of mourning, it is not appropriate to wear Tefillin which glorify G-d’s Name in a public manner. It is the time to retreat and to wait for G-d, in His kindness, to disperse the darkness enabling one to return to Him with love. After this we may reach out to others and light up the darkness with happiness and faith.