Positive: Healthy competition is good, it can give a sense of being part of a team, something bigger than yourself. It also drives people to do better and work harder.
Negative: Too much of anything is bad. It can lead to stressing too much about performance and ignoring health issues for the sake of competition.
I had one experience of the bad competitiveness when I was a first aid volunteer at a kids rugby tournament. The kids were about 11-13 and we were seeing a lot of injuries.(Adult players experience less injuries oddly enough). From bleeding noses to one dislocated lower leg (not pretty). But one fella had a concussion and a bad cut on his forehead and the Dad was in our faces trying to get us to clear him to go back out and play. We basically told him that we reccomend taking him to the hospital immediately for a skull x-ray. But the kid wanted to go back out so off he went.
THAT is an example of competition going too far.