Sorry to hear about your boyfriend,
Don't worry about why, you will spend many nights pondering this but you will never find an answer. You will waste precious time for nothing. Who knows why people do the things they do. You will have to get past this and it's best to start now.
Focus on yourself and what you can do to mend your broken heart. As you make yourself a better person, slowly you will get past this. One day, he may come knocking back on your door, and by then it will be too late, you will have moved on.
If you work on yourself as hard as you worked on him, you will become a strong and independent woman. Someday down the road you will meet the right person for you. You were meant to have him in your life for a reason, you may not see that now, but one day you will know exactly why he touched your life and how he helped you in your future.
Good luck to you.