REASON 1: They know they have to support each other — even if it's obvious one of them might fail.
It happens pretty frequently on the show, but in "Princess Party" Mitchell protests when Cameron will not stop insisting that they allow Fizbo the Clown (Cameron's creepy clown character) to perform at Lily's princess-themed party. But Mitchell knows how excluded Cameron feels about it, so he lets him do it. Of course catastrophe ensues, but Mitchell knows he had to let Cameron do it. It's the same thing that happens when Mitchell decides he's going to build a life-size princess castle for Lily in the backyard, despite his inability to use tools (as in, he has no concept of nail gun safety). Though Mitchell does build a beautiful castle, he ends up trapping himself inside it in the process. Cameron and Jay apologize for underestimating him, even though they know they'll eventually have to pry him out.
REASON 3: They know when it's time to grow up.
Cameron and Mitchell decide to have a grown-up night out with an old drinking buddy, Sal (Elizabeth Banks). But when she essentially proposes killing Lily and calls her Yoko because she broke them up, they decide that margaritas with Sal might not be such a great idea. Though Sal and Lily sort of reconcile in the end, they know it's time to hang up the shot glasses of their past and embrace the sippy cups of their future.
REASON 4: They like to surprise each other.
Mitchell is the stickler of the show, without a doubt, but when he was invited to be part of a flash mob, he decided to use it as a surprise love letter to Cameron to prove that he isn't always so rigid. You really should watch the clip above.
REASON 6: They eventually listen to each other.
Mitchell's plans for Cameron's Leap Day birthday are ruined thanks to unfortunate circumstances (a Wizard of Oz–themed party right when Cameron's family back home experiences a tornado). It leaves Mitchell scrambling to figure out what to do since it's a big one — Cameron's 40th birthday, though he keeps insisting it's his 10th birthday because he was born on Leap Day. The last-minute party on a yacht ends up being a complete disaster, but Mitch makes it up by honoring Cameron's wish to be 10 again. So the entire family skips the janky yacht and heads to a carnival.