Womens breasts are sexy. They draw attention and they excite. When touched right, they make a woman feel good. But, when push comes to shove, nature provided them so that children can be nourished. That will always be the reason they are there.
When a newborn comes into this world, in some way, they need the nutrients that breast milk provide. They must get that somehow. At some point however, they don't need that anymore, and must learn to fend for themselves. Even at that point, some newborns or young children will continue to try and breast feed, thinking that they still need that, even when they don't.
Friendships can also be like that. When we meet people, and as we get to know them, we find that they can give us things that we need, and since most are glad to help, we take that help readily. At some point, just as with the newborn though, we don't need that help anymore. Just as the nature of the mother/child relationship changes and evolves, so does the friendship relationship. Both parties must accept that. The friendship must grow and change, and can't stand on the basis of what formed and maintained that in the past.
In most instances, one or both parties has a hard time accepting that and moving forward into the next phase.
The reality is that they must do that. What someone got from you they may not need at all anymore, or at the very least, do not need it from you. At times, this may mean the end of the friendship, but most times it just means you evolve and the dynamics of the friendship changes.
Failure to realize and put into action this concept usually results in the destruction of the friendship. It doesn't have to be that way, but in many cases it goes that way.