Are all Men Like me or have i just not grown up yet?

Ok i guess both Men and Women could answer this. I am a 45 year old man now. I have been Married twice (and divorced twice). I have had quite a few ok quite a lot of girlfriends. The problem i have is that i find a wonderful lady and we are very happy for quite some time on average 3 to 4 years, then for some reason i start taking interest in other lady's and it starts all over again! Don't say that the lady's start to get lazy as i found that sometimes the opposite happens and my current girlfriend is more attentive as time goes on but i still wander off. I am lucky and have a great job and all the toys that a man could want, i travel a lot with my work all over the world and am mostly very happy with my life but why can i not stick with one woman? Am i normal will i continue to do this all my life?