Well yes and no...
It's a genuine question, I would like to know what peoples thoughts on the matter are, because everyone I have asked this question to tries to deviate and change topic.
It's just dissappointing that the proponents of a theory don't make an effort to know enough about it, always just say "There is too much evidence"
Have you tried to read some of the scientific literature on the topic? There is plenty out there. Obviously asking random people isn't going to get you satisfactory answers.
As for the first point. It is a topic for anthropologists/biologists/geologists (and probably some other fields of science), religions don't attempt to explain the origin of human migration. So this is purely a scientific matter.
As for somewhere to start. I haven't looked much into the subject but it shouldn't be too hard to find some basic articles. Obviously there is wikipedia which sums up the current research to date:
http://en.wikipedia.org/wiki/Early_human_migrations
Human ancestors have been migrating out of Africa for almost 2 million years. Many of them didn't survive, our direct ancestors only migrated out of Africa 100000-200000 years ago. Here is a good article on the basics as well: http://www.sciencenews.org/view/generic/id/69197/***le/Hints_of_earlier_human_exit_from_Africa And:
http://www.ornl.gov/sci/techresources/Human_Genome/elsi/humanmigration.shtml
If you want to read scientific journals on the topic i could probably get some recommendations from people who have studied this area. But most of those are pretty technical/complicated.
In short, there is definitely fossil and DNA evidence to back up the current predicted migration paths. Scientists don't attempt to explain "why" they did it, they just explain when and how they did it. If you want to know "why" some humans left Africa, that is a different topic. There are a million reasons "why" they could have decided to migrate, many smart and stupid reasons. I'm sure there would be some literature on this as well.
I'm no expert on anything scientific though, maybe email your local university for better answers?