Weight

Because when you're the eldest, you have no choice but to carry them- no matter how big they get or how heavy it becomes.

This is probably why Leo's my favourite turtle- I can relate to him being the eldest child in the family. Of course, nowadays, when my siblings mess up, I don't feel responsible. When they get hurt, I don't usually blame myself for not protecting them, because I wasn't there to do anything about it. But then I imagine what it would be like for Leo, and the feelings of protectiveness over my younger siblings must have been so much stronger with him... and so much worse.

Family is all he has. He's a mutant and its only with them that he can be himself. Without them, he'd have no one. So what kind of weight would he bear, if he lived a lifestyle that threatened to take them away from him at any available opportunity? When they had enemies actively trying to destroy them, take away everything that was precious to him, that made him who he was?

As eldest, he is responsible for all of them, and the weight of that must be heavy.

Leo gets a lot of resentment from people because he's too "perfect" or "attention seeking" or whatever. But he's not any of that. The reason he keeps training is because he isn't perfect, and because he can't afford to lose. The need to be perfect isn't what drives him- it's fear.

Oh, look at me rambling. That's what a Leo fan girl does, I suppose, when talking about her favourite topic.

Anyway, in this picture: basically what I've already said. No matter how heavy the responsibility becomes or how tired and bloody he is, Leo will only grit his teeth and bear it. The only thing that's clear to him is that he must carry them; he doesn't know where he's going or what path lay ahead, but it doesn't matter as long as he can keep them safe. He's pretty single-minded like that.

And while he carries them, the other three are free to do what they wish. Raph with his angst, Mikey with his freedom, Donnie with his toys. Mikey, however, is the most perceptive of the three, and he is aware of Leo's struggle. Raph knows too, but he chooses not to see it- he feels guilty that Leo has to carry him and this guilt manifests as rebellion and anger: "Don't tell me what to do! I can handle this on my own! I don't need you!" - even though he knows he's only lying to himself, and that just makes it worse. Donnie, well, he's probably aware of it too, but he's accepted that each of them had a role to play, and he could help lighten the weight by building useful things to help protect them.

Notice how the bowl is open as well. They're all free to jump out and help carry and lighten the weight, but they don't. Of course they all feel responsible for each other (I'm not saying that they don't) but each own's dependency and fears keep them from fully helping lighten the load. As of right now, they all still depend on their big bro, leaving themselves free to pursue other things.

Aha, that was a lot of word vomit explaining the picture. I should have just left it to you to decipher, but I just couldn't help myself. You're of course free to make your own theories and such! I'd love to hear them.