The Bottom Line
A teammate will be the best person capable of giving you constructive criticism.

In Detail
Being a member of a team is good for you -- it brings out your compassion, and provides you with the support system you need to stay positive. And if you need to improve on something, a teammate is the best person capable of giving you constructive criticism. If you aren't currently on a team, that's easy enough to remedy -- all you have to do is join a group that fights for a cause you believe in. Or just sign up for an exercise class at the gym!