What can make friends drift apart?

You know, I have to prepare for my English language exam and to write a short opinion-essay. So, I wonder how many mistakes I've made :D Besides, I want to share my thougts with you and to know your opinion on this topic =)
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Every person has friends. For some, only one close friend is enough, others have two or more friends in their lives. But one way or another, there are some reasons because of which even the best friends may drift apart and will never resume their relationship.
In my opinion, betrayal is the most significant reason. A person, who was betrayed by his best friend, hardly trusts anyone else. But for different people betrayal means different things. One person rates as betrayal the fact that his friend didn’t keep his secret. The other one believes that he was betrayed because his friend had joined their common enemy. I think it’s a matter of moral principles and values.
Another reason why people drift apart is envy. I think, if envy appears in a relationship, sooner or later it will ruin it for good. And if one day you win a million dollars, you shouldn’t be surprised when you know that your friend gossips behind you back and despises you. you in the soul.
Besides, falling in love can separate even the best of friends. And some friends easily communicate with each other even if they fall in love with the same person, but in other cases friends begin to compete with each other and finally they become enemies. Nevertheless, there is another way how falling in love can ruin friendship. Often, when we fall in love, our friends fade into background because it’s more interesting for us to spend time with the person we love. So, some friends may regard it as a betrayal.
There are a lot of other threats to friendship like long distances, changes in people as they go through life, a bad argument; every friendship can face these obstacles. But it doesn’t mean that friendship isn’t worth a place in our lives. Our friends are parts of our souls and if we don’t want to lose them, we should be able to talk over misunderstandings, apologize when we are wrong and, after all, let our friends know how much they mean to us.