Relationships/Cheating

Now, being only 18 this isnt something im a master of, but I feel like I know something about it. To me, when you enter a relationship with someone its because you WANT to be with that person. By some strange chance, every relationship I have been in since June 2010 has ended with me being cheated on. Maybe I just get boring or something, but im usually a pretty nice guy and I am always 100% loyal (like a dog).

So this makes me wonder what drives someone to cheat on the person they are with. Even if its after a few months of dating or a few years of being married, people all around the world cheat on who they "love." So I would like to ask these questions:

Have you ever been cheated on?
If so, did you ever find a reason for being cheated on?

for others:

Have you ever cheated on someone and if so, why?

Considering I just started this Blog around midnight, I doubt there will be anyone to answer, but maybe i'll be surprised and get a few comments.