There's way more stuff going on in the UFC

In boxing, you know what you're going to get in every fight. Two guys wearing heavily padded gloves are going to take some swipes at each other; there will be some shots to the head and some shots to the body; someone could get knocked out or the fight could be stopped due to a beating/injury/infraction. More than likely, though, two uninterested dudes are going to dance around the ring for 30 minutes and barely ever engage, leading to a judges' decision that's probably the product of illegal coercion, and the dirt bag promoters already got paid anyway, so the joke's on you, sucker. Not the case in the UFC, whether the styles of the fighters match up or not (more on that in a second), you rarely know what you're going to get. First of all, the nature of the Octagon makes things crazy all on its own. Second of all, outside of a few key common-sense rules -- no fish-hooking, no eye-gouging, no crotch-kicking, no biting, etc. You can pretty much harm your opponent in any way you so choose. 

Nontitle and title fights respectively. The point? Most of the time, you've got a quick 15 minutes to either beat your opponent into submission or convince the judges that you just gave him more brain damage than he gave you. You can't coast for three rounds, then change your game, then back off once you have a slight advantage. This makes for a much better fight structure because it forces action, you lose a round in the UFC, your back is against the wall. You've now got to throw it all out there. In MMA, every round, every tactic, every second means something. You screw it up, you don't really have another chance. The fighters know that. And it makes them crazy people.