I've had previous best friends get engaged before, and inevitably they go off and live life. Married life. I'm not going to say they forget about you. Of course not! Not if you were really good friends. It's just different. Just as their lives will never be the same, your relationship with your friend will never be the same. They're the same person, but entirely different. You're not going to be driving around at all hours of the night getting slurpees or ice cream or whatever. You're not going to be swapping dating stories anymore, because they won't have any. Really non of it will be the same. Your conversations will be completely different, and you will be spending virtually no time in person anymore. They're still a friend, but they go from "best friend" to more of a really good acquaintance.
If I sound bitter...that's because I am. G-d forbid I'm not blaming anyone. That's just the way that life works. I don't expect you to go out with me when I'm feeling bored, because thankfully you're not bored. You've got a spouse to take care of. I don't expect you to talk to me as often, because you have got someone more important to speak to. I wish I did, and I'm not planning on acting any differently when it happens to me. Every time you get a text in the middle of our conversation, and you laugh and answer back, I'm not upset at you. You haven't done anything wrong. I'm just reminded of that which I don't have. That which seems so far away. That's not your fault.
But just because I'm bitter doesn't mean I'm not happy for you. I'm really excited that you thought of a great way to propose, that you found this perfect gift that she will love. I'm really happy that you're happy. I love that you have found someone. You truly deserve it. You haven't had an easier time than me by any stretch of the imagination.
However, there is a difference between being happy for you and being happy about this for me. I wish I could do both, but right now I'm only the former and not the latter. And please don't blame me for this.