I remember the pure happiness and joy I felt for mitzwoth, but I couldn't begin to put it into words. When I was speaking to my father on the phone last night, telling him about this joy, I found a way to express it: The pleasure and joy that we can experience with each mitzwah is the same pleasure and joy we experience through our children.
I could never have understood this before I became a father, and still there are many levels which I have yet to experience and so, I am yet to understand. But, the pure richness of joy a parent experiences, the nachas that a parent feels at seeing their child cheerful/successful/etc, the awe that a parent feels whenever their child does something new, that is the pleasure and joy and relationship that we can (potentially) have with our mitzwoth.
[This expression lead me to consider and understand the following only today:]
It is, perhaps, for this reason that Chazal explain that the mitzwoth of Tzaddikim are called their toldoth, their generations. (אלא תולדות נח, אלא תולדות יצחק בן אברהם)
Whenever we do a mitzwah, it can be a chore, it can be a responsibility, it can be a priviledge, it can be a treat, or it can be giving birth to another child which we nurture and care for, which fills our hearts with love and joy until we feel unequal to the task. Our mitzwoth don't really leave us and vanish as soon as they are performed, they stay with us and add more light to our life. כי נר מצוה ותורה אור - the mitzwah is like another candle that makes our world [a lot] brighter--forever. When we learn Torah it increases the light that shines from our mitzwoth, not only the one's we have yet to perform but even those that are behind us.
This is a form of Teshuvah we don't normally consider, making our (past) good deeds better. When we keep our mitzwoth with us, and nurture them with more Torah, when we know they are eternal and are forever with us, even after our bodies have long vanished, we can improve them even after the act is long done.
It is only when we let go of our mitzwoth, fire and forget style, that we separate ourselves from them. Just as a parent could, heaven forebid, turn their back on their child. Yet, all is not lost, all is never lost, just as a child always longs for their parents, so too the mitzwah always longs for us to revisit it, to be nurtured and fed, to grow.