Can I prevent freezing?

Yes, there are some preventative measures.  You want to try and speed up the computer.  You can do this by trashing some things to free up space on your computer.  You can add more memory -- but that costs money.

You can run programs that "defrag" the computer like Disk Warrior. Defragging is just a fancy term for organizing all your data. Think about it as if you are trying to get through your child's room but it is all messy with toys haphazardly placed around the room. Defragging is like asking your child to clean up his room. Once it is straigthened, it is much easier to get around. The computer is the same way.

On the Mac, you can also try resetting the PRAM. To reset the PRAM, restart your computer and hold down the option + apple + R + P (make sure the caps lock is not engaged). Hold the keys down until you hear the start up sound a second time, then release the keys. This will reset any special setting you had in your control panel.

Why does it freeze?

A computer freezes because it is trying to handle too much information all at once and it gets confused.  Usually, it is running slowly and you keep clicking the same thing again and again.  The computer then tries to execute all of those commands and eventually gets overloaded.