I can speak from experience. D:
It's not a nice feeling to have someone whom you trusted and loved oh-so dearly simply leave without a trace or explanation, but the only thing you can do is move on. Unless you're persistent in trying to keep in touch and manage to get a hold of them and truly want to seek out a reason, then I wish you good luck, but the main idea is to carry on forth. Perhaps it could teach you something - you never know! Pick up interesting hobbies, meet new people... It's tough to linger in the past, as bittersweet as it is... (: There are so many other people eager to meet so many other people. You'll find someone (or find this person again) in no time. Patience is a virtue, after all.
