You cannot count on anyone except yourself.
(A nod to Carolyn Burnham, from American Beauty)
I mean this in the best way possible! I learned this lesson in high school (no surprise there). I went on a three month exchange, and when I got back, [the people who I truly believed to be] my friends casually remarked, 'Oh, did you go somewhere?'
Even now, in the 'real world,' everyone has their own little life, with their own priorities, and their own obligations. It's not being selfish - it's just reality of being individuals. Ultimately, if someone had to chose between themself or you (to eat, to drink, to live), 11 times out of 10, they'd pick themself. But so would you.
I think what I'm trying to say is that you will live a fuller life if you face certain mortal realities: family members will pass on, friends will come and go (they may even come back), but life keeps on moving. And life will keep moving long after you are gone.
'We're born alone, we live alone, we die alone. Only through our love and friendship can we create the illusion for the moment that we're not alone.' (Orson Welles, 1915-1985)