Let's talk Spell casting for a moment?

I was looking through the books the other day and reading about how magic users learn their spells and cast them. Each level has a "cap" to how many spells a MU can cast a day per level or should I say, can memorize and then cast per day. Now any spell found or gained over the max level of spell caster, is considered to be just a spell read off a scroll. The question that was flying around in my big empty head was, what is to stop a 4 level magic user from learning to use a spell that is much higher in level then he is used to? Is it the magic itself, not allowing a MU to learn that spell, because they haven't gained enough experience? Well, one would think, at 4th level, a MU would be experienced and confident enough to easily grasp a spell, memorize it and cast it.

Is the length of the spell the reason why? Why can't a 4 level MU, every night after studying his spells, read the higher level spell and break it down so he could make himself learn it? What's to stop him? I don't see a real valid reason, other then its higher level and he just can't. I've heard its just the complexity of the spell. Well with a 17 or 18 intelligence, one would think that character after sometime, would figure out that complexity and learn that spell.

Anyhow, just some food for thought.