This question is for both guys and girls.
Most Helpful Guy
The simplest reason: because you want to break up with that person. No further reason beyond that is required and neither you nor your significant other has an obligation to stay in a relationship with you. Justification for making the decision to break up is not required for the break up to happen.1
Most Helpful Girl
All relationships have their bumps and your partner will annoy the heck out of you at times no matter how much you love them... But if you are unhappy and honestly cannot see things working out in the long term, no matter how hard you try, then that is a good enough reason to let it go. Holding onto something and hoping it'll change is doing nothing but hindering both of you from having a fulfilling life.0