An overprotective boyfriend's behavior comes from his insecurity. If you've been dating a man for a little while and he quickly starts to move the relationship forward, telling you he loves you, trying to get you to move in and insisting on seeing you all the time, then he is probably going to be overprotective. Moving a relationship forward quickly shows that he wants to "lock you in," as he is too insecure to have faith in his ability to keep you interested over time. Insecurity breeds overprotective behavior.
An overprotective boyfriend will make everything about him. Whenever you do an activity, it will be an activity of his choosing. This is a sign of controlling behavior, which is another component of overprotective behavior. Your boyfriend will choose all the activities because he is either too selfish to think about what you want to do or obsessed with being in charge all the time. Either way, it can quickly lead to overprotective actions.
If you find your boyfriend slowly (or quickly) pushing other people out of your life, it's likely he is overprotective. Not only will he be jealous of other guys, he will also be jealous of your family, friends and acquaintances. He will be afraid of the influence they have on your life and will not want you to be involved with them. He will become upset when you do things without him and constantly come up with activities that don't involve your friends.
While some men can be sensitive, overprotective men are extremely sensitive. He'll respond negatively to every slight, whether it's real or imagined. What's more, he'll respond disproportionately. If you tease him a little bit, he will get extremely upset. If you make plans that don't involve him, he'll also get extremely upset. Essentially, an overprotective man is too insecure to keep his emotions stable, and a hypersensitivity is a good warning sign of things to come.