It's disconcerting when a relationship appears to be humming happily along -- until it isn't. If your boyfriend's sudden decision to be single has left you wondering what the heck happened, you're in good company, as this is a common scenario. That doesn't make it easy, however, and you'll ultimately have to make your best guess and try to avoid a similar situation in the future.

He Couldn't Share His Feelings

It's possible that your boyfriend has had misgivings about your relationship for some time now, but he felt uncomfortable sharing his feelings. He may have worried about what your reaction would be if he told you that he is bored hanging out with your friends or would like to spend more time pursuing his own interests. It may have been easier for him to break up with you rather than gather up the courage to address the aspects of the relationship he was dissatisfied with.

Negative Interactions Outweighed the Positive

Marriage researcher John Gottman found that what makes the difference in a relationship is a "healthy balance between their positive and negative interactions," according to The Gottman Institute. In fact, for a relationship to be a happy one, there should be five times as many positive interactions in your relationship as negative ones. You don't have to be married to experience relationship failure due to negative interactions. If negative comments and depressing experiences were part and parcel of your relationship, he may have simply decided the negativity wasn't worth it and cut his losses.

You're Not Who He Thought You Were

People bring a delusional mindset to relationships, that is, they think you are someone you are not. Over time, perhaps they will grow to like who you really are, or perhaps not. If your boyfriend suddenly gave you the heave-ho, it may simply be that he realized you're not the perfect, adoring, agreeable person he thought you were. This is not your fault and does not reflect badly on you in any way. It just means that you're you, not the imaginary person he superimposed on you at the beginning of the relationship. This type of thinking leads to a "nightmare on romance street," according to the dating website eHarmony in the article "Nine Reasons Good Relationships Go Bad." If this is the case, embrace the breakup and be happy you avoided a drawn-out nightmare.

The Thrill Is Gone

If your relationship was based more on lust than shared interests, he may have realized that there was nothing to keep him with you once the thrill of conquest wore off. Intense sexual attraction can obliterate common sense, says psychiatry professor Judith Orloff in the "Psychology Today" article "Lust vs. Love: Do You Know the Difference?" If he entered the relationship thinking with his loins rather than his brain, it's no surprise when he then avoids true intimacy, choosing instead to move on.