My cousins are drifting away...

I always notice that the people I have close relationships always end up drifting away. There’s always the height of our time together and then eventually after a year or two things change. I lost a friend that way and we just one day stopped talking. I never thought it would happen with my cousins (they are not blood related except our half brother is related to them). It might be because of the age difference. But either way I don’t find it worth it anymore. I’m just tired of being the one side of the conversation. I’m tired of having people never being there for when I need them. And if I break away they just give up too. How come things never last? Is there nothing to do to fix our relationships?