Parallel relationship when the lover is staying far!

I thought this happens only in movies or in some rare occasions in real life. But I'm now seeing it happen in front of my eyes.!! I always thought love is something that must be pure and committed and enjoyed naturally. But why do people tend to divert and find someone else if their partner is staying far from them for some reason like job or education.? Does a short time physical absence fade our love? Our actual love? Some people either completely break up with the old love or they play the game both sides! I just hate it. Alright, a long-term absence of our partner is a different matter. But when you are truly in love and committed ( lets forget if engaged officially or not ) how on earth can your love shift to someone else who is nearer? I agree, the physical absence hurts and we don't feel good, but how does it end the love for the person? IS THIS WHAT LOVE IS ALL ABOUT? I feel so strange and I can't believe it!! Falling in love again after breaking up with one is acceptable. But if the love is true then it must be natural for anybody to be thinking like that only about one person - whether he/she is far or near! What do you guys think?