Because we have the beauty and wonder of being complex beings with amazing minds and myriads of different experiences. Feeling a mix isn't necessarily a bad think. Taking my son to Kindergarten the first day for example. I was so proud of him and happy for him to reach that milestone, but at the same time, my heart was aching because I missed my "baby" and had to let others be the ones to care for him during the day.
The same "baby" is now in high school and in a few years will leave to go to college. He's worked very hard academically and I couldn't be more proud and ecstatic for him and his accomplishments, but the day he leaves I know I will be heartbroken too, because I have to let him go again.
I wouldn't trade either of the feelings for the world. They both add a depth and richness to life. Could we really truly experience and appreciate joy without sorrow? They are opposites, but they are both necessary parts of experience :). As for grief, the deepest of all sorrows, it is painful but also healing when we move through it instead of pushing it away. I guess I've seen enough tragedies in life that I've come to appreciate that sorrow and joy are both intertwined in many ways. I can reminisce now about those I've lost and feel both - although over time it's more happy than sad thankfully.