Let's just say that you used to like somebody, and then you told them you liked them, and they don't like you back, and that's fine and you're both still friends.
Now let's say that this crush doesn't go away and instead only develops further, and when they even mention the fact that they had a partner for like a month you get depressed for a full week or so because you've already gone through the first step of acknowledging that they don't like you, but you can't accept the fact that they can like other people. This is hypothetical, by the way. Wouldn't happen to me in a million years.
And because you felt like crap because they have a life outside of you, you feel like an asshole because you're only having feelings that go against their own wishes and you're scared to broach the topic again because you've entered a neat stage of non-acknowledgement, and you thus start freaking out whenever you even talk to them because you think that they might have a new partner and you'll get that depressed again. And also, you start freaking out when you see / hear them do anything at all, and stuff.
This is a hypothetical situation. I'm not going through this. But if you have answers for this made up scenario... Help?