He is not ready to commit
This may be a no-brainer. When things are going extremely well, and every day is sunshine, a sudden onset of wintry weather means one thing: That he was not ready to stop playing the field.
The hints you may have been dropping, like telling him about all your friends getting married and having kids could have scared him. Commitment, after all, is a choice. When people do not want to commit yet, they will absolutely not force themselves to do it.
Unless you want to set yourselves up for a dissatisfying marriage and divorce, that is. Neither of you is at fault for this. Preparedness for commitment is not something that happens overnight. Do not blame him for not making a mistake.
One of you loves another
I am not talking about cheating. What this means is that one of you has developed feelings for another person, and the other can sense it. Men are much more likely to simply cut ties with you because of this.
Women, on the other hand, will oftentimes try to dismiss these feelings, leading to awkwardness in the relationship. Do not blame each other for the change. If you fall in love with someone else, it simply means that the two of you were not meant to be.