When does sharing someone become competing for their attention?

All relationships are give and take - friendships, marriages, kinships - they have to be in order for them to work. Going into a relationship knowing balance is necessary can prevent some of the pain that is often associated with relationships. When we were kids we tended to horde our friends and hated sharing them with anyone; as we grew older most of learned that it was impossible to be the center of everyone's universe (some people still havent gotten the message however). The problem becomes however, how much sharing is too much? When it comes to relationships, serious or not, when is it ok to say I'm sharing you too much?

In my relationships, I have always been more that willing to share the people with whom I associate; i like to think I do a good job picking my friends and if they are good enough for me, other people should be able to benefit from there company as well. My problem comes when I have to share someone with material things. Hobbies are healthy, but when they become obsessions and affect our relationship is when I don't have time. I don't need to share you with with your car, your house, or your internet addiction. If you are not smart enough, mature enough, or aware enough to realize that what you are doing is wrong then we need to move on.

I am not going to compete for attention.