Pastor Tim Keller explains why Jesus is the reason for self-forgetfulness:
There are two basic narrative identities at work among professing Christians. The first is what I will call the moral-performance narrative identity. These are people who in their heart of hearts say, I obey; therefore I am accepted by God. The second is what I will call the grace narrative identity. This basic operating principle is, I am accepted by God through Christ; therefore I obey.
People living their lives on the basis of these two different principles may superficially look alike. They may sit right beside one another in the church pew, both striving to obey the law of God, to pray, to give money generously, to be good family members. But they are doing so out of radically different motives, in radically different spirits, resulting in radically different personal characters.
Read the whole thing. Keller unpacks this theme more fully in his excellent book released earlier this year, The Prodigal God.