Just as our bodies are shaped as a result of a combination our genes (height, build, and likelihood to put on weight) and the environment (what we eat, how much we eat, how much exercise we do). Our psychology, and how we behave is also a result of our genes and our environment combined. If you don't educate, socially condition and teach a human being, then it will pretty much act like an animal. Most of what we know to be right and moral we've learnt from society and what our families have taught us. But it doesn't matter if we try to learn, our basic instincts still remain and are often in conflict with what society teaches us, and often control us.
Guys are pretty simple creatures when it comes to relationships. We are diven by sex. Not only that, guys are driven to have sex with as many women as possible. Now before you say I'm wrong because you know good guys, and married men that don't cheat, that's doesn't mean those guys don't fantacise about sex with other women, they just don't act upon it because society has taught them it's wrong to cheat. Also, they know that if they cheat and are caught then they will probably lose their partner for good. I got two perfect examples of smart successful guys that got caught doing what their basic instincts drove them to do:Bill Clinton (US president at the time he cheated) and Elliot Spitzer (Governor of New York state when he visited prostitutes)......If you don't know what Bill Clinton or Elliot Spitzer did then please look it up on wikipedia, it's all there.
Now these guys are smart, and knew besides the fact that cheating is wrong, that if their political enemies and the press ever found out what they were doing they would be politically crucified, and embarressed in front of the whole world. Yet they did it anyway. So what drives men to have sex on their minds all the time? According to research most men cum in 2-5 minutes. So it takes 2-5 minutes for a guy to make a baby, whilst it takes 9 months plus....... 2-5 minutes for a woman to make a baby. That's simple fact is the reason why men and women approach relationships so differently. Think back when we were still "cavepeople". The guy that has sex with the most women will have a higher likelihood of having more offsprings. This means there will be higher likelihood the guy will pass on his genes, and this behaviour is past on to his offspring.
A couple of million years later, when human society has rapidly developed this behavioural need to have more offspring remain, even though we've learnt how to use contraceptives. So what you basically have are horny guys that don't have dozens of kids. It's why you have guys that sleep with a girl and then moves on, it's why some married men cheat, and it's why prostitution is the oldest profession in the world. So at this point a girl would cut in and ask, wow great, all men are scumbags that will sleep around.
So how does that help me find a nice guy? My reply would be, do you even want to find a nice guy? I'll explain why women might actually not want nice guys in part 2.