You have these two friends. You love them dearly. One of them you have known for years and the other is new and shiny. One day you all find yourselves in the same place at the same time, and your two friends get to know one another. Sometime later you realize that they have really gotten know one another well. They spend time together chatting and laughing - without you. They confide in one another - not in you. They start to hang out together - without you. They do stuff together - stuff that you aren't good at. They have a lot in common - things they do not have in common with you. They are younger and hipper and smarter than you, and their conversations seem out of your reach at times. They seem really happy when they are around one another. And you find that you are not feeling so happy at all.
You know that you should be happy that you were instrumental in helping two people find one another, that you brought two people together in friendship. After all, one cannot have too many friends. You know that a mature and healthy individual would either find a way to join in the fun or wish them well and move on. Instead you hang back and feel like a third wheel. You feel left out.
You're not sure which is worse - feeling like your long time companion has replaced you with a new toy or feeling like your new friend has already grown bored with you. At times you regret ever introducing them. You are reminded of all the times you weren't interesting enough or pretty enough or smart enough or funny enough. Of all the times you just weren't enough.
It's no one's fault, really. No one can help who they like. No one means for this sort of thing to happen. You can't say anything to anyone, because then you would be acting like a baby. No one likes it when grownups act like babies. So you give in to jealousy and resentment and loneliness. You keep it to yourself and hope that it will pass soon. And you vow to keep all of your friends away from one another from now on.