Your relationship was going so great and suddenly it seems like you're alone in it. You don't know what you did wrong. Here is a list of things that could be causing him to pull away from you and what you can do about it.
He loves the chase
Whether consciously or subconsciously, many men love the steps leading up to a relationship: catching a girl's attention, winning her over, etc. But once she starts to have interest, he loses interest in her. This doesn't necessarily mean the guy is shallow and just interested in games; but it could mean that you based your relationship around building interest in the chase instead of building interest in you.
When you start dating, make sure you let him in on what an awesome person you are so that he knows he's attracted to you and not to the chase.
You're being too clingy
Of course you're going to get pretty attached to your guy, but being overly attached can hurt your relationship. He needs to have space to breathe, to be himself. Not only that, but he needs to know that you're a high-value woman who doesn't need him in order to be secure. That's the kind of person he wants to be with.
His life is stressful
Sometimes the reason he is pulling away has nothing to do with you. He may be overwhelmed with family or work issues that are preoccupying him and causing him to become distant.
In this case, it can be good to give him some space so that he can deal with these challenges. However, don't get too far. Make sure he knows that you are there to support him in whatever way you may be able to. He might need someone to help him with smaller tasks or someone to listen as he vents his struggles. You can also do small acts of service for him to help his days be better.
You may need to be OK with carrying the bulk of your relationship on your shoulders for a time. However, if this lasts a long time, you should consider the fact that this could be more permanent than a phase, and it's not healthy for you to continually be contributing to the relationship without getting anything back.
Commitment terrifies him
In this day and age, commitment issues are pretty common. He may be scarred from past relationships or have a fear of missing out on something better. If you felt like you were progressing with a guy and he is suddenly pulling back, he may have some challenges with commitment. Make sure you talk and try to resolve what is challenging him.
When you fall for someone, your brain releases chemicals that give you feelings of excitement, increase your heart rate and make you act "crazy." As time goes on, the release of these chemicals lessens and your love is forced to feed on compatibility instead of just chemistry. If you and your partner haven't built up your compatibility, when the initial rush of lust wears off, your boyfriend may begin to pull away.
To avoid this, make sure you develop your relationship by resisting the urge to get physical too soon and focusing on getting to know each other.
He fears the end
Some men become terrified of losing a good thing or being broken up with, so they prematurely pull away as a self-preservation mechanism. This can be overcome with communication. Make sure you honestly express your feelings to your boyfriend and help build his trust in you and the relationship.