Lets first talk about captain planet, so if you don't know it, it's a cartoon from the 90's about a super hero that protects the earth. he is summoned by the planeteers combining their powers, earth, fire wind, water...and heart. Also i would like to note that all of the planeteers were from different parts of the world. It was a kick-ass show that i watched all the time as a kid and it had a good message. unlike a lot of cartoons that are around today, in fact if we compare the cartoons we watched as kids (eg gargoyles, transformers, GI joe, batman( which i think is one of, if not the best adaptation of batman)) to the cartoons they have today, it just doesn't stack up.
Getting back on topic, Ma-Ti. he was given the power of heart while the others got powers like wind, water, earth and fire. Now i know, without heart then captain planet wouldn't be the super hero that he is, in fact i think they even done a couple of episodes addressing this but still if i was Ma-Ti and looked around at my friends doing all of these amazingly awesome things with there powers and i had heart as my power, i would feel at least a little disappointed.
I am pretty sure that is why he got a monkey. Monkeys rule!