There's nothing wrong with them, they're just....not a good fit. You and your friends have a certain chemistry that the Self-Inserter doesn't mesh with. When you suggest pizza for dinner, they say they want Ethiopian, and since they're the one sleeping with your buddy, guess what you order. The Self-Inserter is far from the worst person your pal will date, but they do serve as a reminder that just because you like the same person, doesn't mean you're going to like each other.
Your friend talks about this person constantly, you're friends with them on Facebook, and you've hung out with them six times. That said, you couldn't pick them out of a lineup with a gun to your head. The Dud is boring, forgettable, and, for some reason, your friend is obsessed with them. Dating them is like saying your favorite food is mashed potatoes: They're not bad, there's just so many more interesting options out there that it's perplexing you'd settle on that.
You know your friend, and you know what they're looking for in a mate: The Short Termer isn't it. No matter how nice or fun or hot they are, you already know they'll be gone by the end of the month. Just a reminder: There's nothing wrong with having fun with someone, but it's never a good idea to bring the person you're having fun with around your friends. It's impossible not to look at them like they're a lamb that's about to be slaughtered, making them very hard to even try and have a conversation with.
You know your friend and you know what they're looking for in a mate: The One You wanna Be Friends with also isn't it...BUT WHY THE HELL NOT?!?!?! They're so cool. They have a cool ass job, a kick ass dog, and they're just generally fun to be around. The fact that they're dating your friend automatically makes the two of you friends, and that's badass. Who cares if your buddy doesn't see a future with this person? Can't they just take one for the team and keep their sham of a relationship going indefinitely?
The Grown Up relationship is probably the best person possible for your friend to date. Once he starts dating the Grown Up Relationship, your friend begins acting responsibly, thinking about their future, and, for the first time since you've known them, they actually seem like a true adult. In short, they get boring. For as good as The Grown Up Relationship is for your friend, you can't help but resent them just a little bit. Your friend used to be fun, now they go to bed at reasonable hours and drink merlot. Sorry, but nobody wants to hang out with two people in a healthy, adult relationship. Yuck!