just here for the stimulating conversations

Mikey and Donnie have their moments too, but in this particular episode, they're in the background as they witness another falling out between their leader and the resident hothead of the family. They didn't ask for Raph to take over, and no sooner than later do they realize Raph didn't know what and all he was signing up for when he freezes up in the middle of the fight. Donnie is the one to formulate a quick plan of action as he snaps Raph back to reality and incapacitates Snakeweed long enough for them to make a swift exit from the hideout, while Mikey remains unconscious for nearly the remainder of the episode due to Raph's poor planning.

And I seriously need to stress what exactly Splinter tells Leo: "Leadership is not about being appreciated. It is about responsibility. It doesn't matter that the burden is heavy. It matters that you carry it. Now go find your brothers."

He never said Leo was wrong for seeking appreciation; in fact, his future words and actions support the guys learning to show appreciation to Leo, and Splinter is understanding everytime Leo expresses his distress over being responsible for his brothers when they love making his job more difficult than it needs to be. But he needed Leo to understand that when it came to completing a mission effectively without unnecessarily endangering each other, appreciation holds little importance in the moment. You can't use appreciation as a deciding factor for whether or not you should let the team fall apart because your younger brother stomped his foot at you for the umpteenth time.

Splinter emphasizes that leadership isn't something to be taken lightly, and Leo needs to realize it's not something you can just pass on when you aren't in the mood to hold it.

Now, one can argue that it does matter to some extent that the burden of leadership is heavy and their argument would be valid; they're still kids after all and the burden may gain some pounds depending on the situation. That's why a leader leads a team though, so that the weight of that burden can be shared by the members and their efforts when the leader starts to buckle.

But, once again, Splinter is right. It does matter more that Leo carries that burden, or at least the most weight, because he's the leader. Not only are they at a point where Leo is permanently cemented in place as the leader, and things would undoubtedly be shaken up in an undesired manner should he thrust someone else into that position this far in, but his brothers aren't entirely cut out for leadership. Planning ahead doesn't come easy to Raph, Donnie gets tunnel vision and misses details that would be obvious otherwise, and Mikey doesn't always think his actions through before doing them (a bit of blind faith if you will). That's not to say they can't defy the odds at times, but in the grand scheme of things, they function much better as a team with a leader, and Leo functions much better as a leader with a team.

Team building exercises aside, the way Splinter speaks and phrases things gives the same vibes as a parent telling their child not to make a habit out of something they shouldn't be doing. He was evidently upset with Leo for allowing his frustrations to do the decision making for him, but he specifically wanted Leo to not make this a recurring thing because they won't just fall apart as a team by that point: they'll fall apart as brothers. Giving someone what they want because they irked you into doing it can establish a pattern of behavior that no one wants to deal with the consequences of, and irking someone into giving you what you want isn't something you should do either. It sucks, believe me I know, but Leo shouldn't just let his brothers get their way because he's fed up with them, and Raph shouldn't always get his way because he doesn't agree with Leo's way of doing things.

Doing that creates the opportunity for something bad to happen that could've easily been avoided, similar to letting a child do something you know they should be broken out of before that something becomes a problem in the future.

And Splinter doesn't seem mad when the guys come rushing back to the lair without Leo in tow, he sounds more disappointed than anything. As I mentioned earlier, he knows the others give Leo a hard time and he's aware that leadership is no easy role to be sworn into, but he was hopeful that his earlier lecturing would bring Leo to his senses and his son would prioritize responsibility over running off to only God knows where. He doesn't linger on his earlier conversation with Leo though, he spends a few seconds to scold Raph for instigating this entire mess in the first place. He makes sure Raph is aware of the pressure Leo has on his shoulders, with an underlying warning to not take leadership so lightly again, and orders him to find Leo.