1. "Bros Before Hos"
Situation: Guy A likes you. He is a decent guy, and when asks you out, you decide to give him a chance. You go on a couple dates, but eventually you realize he isn't quite your type and you relegate him to the friends zone. You can tell he is disappointed but there is never an explicit "break up" since there was never a real relationship, just a couple dates. Before long you are hanging out as friends, in larger groups, and eventually meet his friend, Guy B. Guy B is attractive, and even flirts with you, but never makes a move. Maybe he is a little more overt with his affection when he is drunk, or flirts more with you when Guy A isn't around, but the status quo always returns and you get discouraged and confused.
Explanation: Guy B likes you - maybe even a lot - but he isn't making any advances because it would be humiliating to Guy A if his friend ended up with the girl that he wanted but couldn't have. While he wants to be with you, he places more value on a long-term friendship with Guy B than he does on a potential girlfriend - however much he thinks he likes you at first glance. In a guy's world, the two relationships are mutually exclusive.
2. Most Guys Won't Cross Swords
Situation: You got drunk at a party/bar and slept with Guy A. The next week you are out with a similar group of friends, but Guy A doesn't come out because he is away for the week. Guy B, however, does show up. He was also there last week and is actually much hotter than Guy A, but had a girl with him last time and wasn't paying attention to you. This week he flirts with you a lot but doesn't ask for your number or respond to your strong sexual advances. You go home alone, feeling insulted.
Explanation: Guy B wants to bang you, but he was there last week when Guy A told his buddies about having sex with you. Not only is Guy B kinda creeped out by the thought of his junk touching the same vagina as his friend's, but you slept with his buddy first, so he feels like he is second best if he hooks up with you now, after his friend.
(NB - For some reason, enlisted military guys do not care about this. They will actually fuck you at the same time - maybe even touch dicks in the process - without a second thought.)
3. He Pointed You Out First
Situation: You are in a bar having a beer with your girlfriend when Guy A and Guy B approach the two of you. They are both cool, but Guy A is hitting on your friend while Guy B is talking to you. This is a problem because not only do you think Guy A is hotter, but you know that your friend actually prefers Guy B, so in your minds, things are completely mixed up. Although you try a couple times to change the conversation so that you can chat with Guy A, the guys eventually bring it back. You end up giving your number to Guy B when he asks, and Guy A takes your friend's number. It's great that you met them, but you wish the pairing had been the other way around.
Explanation: Guy A and Guy B saw you and your friend from across the bar. Guy B pointed you out to Guy A, and suggested that they go together to say what's up to you. Guy A had actually seen you two at just about the same time as Guy B, and was more attracted to you than he was to your friend. In fact, he probably would have hit on you himself if he'd seen you a minute earlier; but because Guy B already had his hopes up about meeting you, Guy A encouraged Guy B, and played wingman with your friend (he asked for her number in front of you so that you would be more receptive about giving yours to his friend). Since the guys' initial conversation established who was going after whom, they both actively fought to maintain the conversation pairing when you attempted to change it. While Guy A sacrificed an opportunity to get a date with a cute girl (you), it was more important to him that he support his friend, who pointed you out first.