We are put here on earth to learn one thing and that is love. Some learn it faster than others, and we are called to teach each other how love is the way to God.
The ego gets in the way and demands its due. It makes us become defensive, selfish, greedy, self justifying and yet there is that little voice of the Holy Spirit that calls us in another direction.
In the spiritual life, we listen for and respond to the Holy Spirit's call to love. We hear the call in our minds. Hearing the call in our mind leads to movement in our hearts and bodies. As the call moves from our minds to our hearts and bodies we reach out and bless all that we are in contact with.
In the process of blessing others we also bless ourselves so that blessing is a win/win situation and humanity accelerates through time to the kingdom where we will experience joy forever.
Joy comes from love, and love comes from blessings, and so when we say "bless you" we extend our holiness and love around the world beginning with the person next to us.