Relationship Endings!!!

Its valentines week and I do not have nothing much to do :) So I am going to write about Relationship Endings! I have a very good friend who was in a relationship when we were in our graduation. Both of them were visibly interested in each other. Friends played cupid and the love blossomed. Everything was hunky dory for sometime. I don't exactly remember how long the relation worked but say couple of months atleast. Then it happened the guy felt that she is being too possessive. Symptoms being she couldn't stand him talking to any other girl in college. She even suspected her best friends intentions. My friend seem stuck and helpless in that non functional relationship. Her suicidal threats were more than enough motivation for us friends to finally help him with getting out of that relationship. We helped the guy get out of it with a lot of drama involved but no one hurt in the process except for the girl's heart I guess. But anyways she found a sweetheart within couple of months from our class itself !
Now this friend got into a second relation after sometime. This relation went on for quite a long time almost 2 years. I was not a cupid in this relationship so I hardly knew the girl. Both seemed serious about the relationship. Everything seemed nice, but they eventually broke up and the break up was messy! I didnt personally witness any but what I have gathered it was very hard for the girl. She couldn't come to terms with it for a long time and she tried to hurt herself just for this relationship. Anyways I recently came to know this girl better and I realised that she is a gem of a person. An actual sweetheart the ones you would proudly take home to your parents! 
So I asked my friend, who is single right now, Why his relationship ended with such a nice girl?  His answer was "Yes I know that she is a wonderful person, I am still to meet someone so dedicated to only one person, I have met a lot of girls but not like her, but I was bored!!!"
That answer rattled me, but I guess that's a fact.... People get bored and they lose interest and they don't see any value in holding on to that relationship. That's when they call it quits! 
Still I can't seem to understand  How can they forget the time that has been shared?  How can one person become so selfish so soon? Is everyone capable of ending relationships so fast?  
Something to ponder over  Should there be a notice period in personal relationships too?