Here's my understanding of defenses (I could be wrong, but here goes):
... is exactly what it says: how defensive your build is. Pokemon like Aggron are quite literally steel clad, so that's high defense stat. I'm going to make a lot of comparisons between real life and Pokemon, so a Pokemon with high attack is similar to a predatory animal or an overall reckless person.
Reckless people often don't care what happens to what happens to their own body, so long as they are getting their job done. They leave they're defenses wide open in hopes of landing hits, giving all their focus to offense. This is the most likely explanation as to why Pokemon with high attacks have low physical defenses.
... on the other hand, would probably indicate mental strength. Prior to Gen 2, we had just one "special stat" which indicated psychic (or mental) power. After the split though, it's safe to assume that Special Attack is one's power to influence someone (or something) with your mind, while Special Defense is your power to withstand this control. Or your willpower.
Now I would proceed to compare a Pokemon of high attack stats to our same old reckless person. Most reckless people are very competitive people, who alongside having strong means of executing their ideals also have a good deal of willpower. This is commonly observed in competitive game players.
For example: one of the marks of a good competitive chess player is his ability to bounce back from an otherwise devastating loss and play his next game at near prime condition, if not better.
There, you have my two cents. Of course this may have a few holes around it, but it's the best I could come up with. Comment if you disagree anywhere.
Hope I helped!