This is a theme that is in any pixar movie. For example, Monsters Inc. when Sulley was conflicted with being a monster that is supposed to be scary and being nice like telling jokes. At the end as I am sure most of you have watched it saw that he made a new company where the monster tell jokes instead of scaring the kids. Another example is in Toy Story when Woody realizes that Buzz is the most popular by the toys and Andy. He tries to leave the house and realizes that he isn't that bad and popularity isn't all that matters.
Don't be selfish, help your friends when they are in need
This the main theme in Toy Story, Finding Nemo and The Incredibles. In Nemo, Marlin wants to go by himself to get his son but knows he can't because of the dangers in the ocean. Marlin would never get to Sidney if he didn't have Dory around and it wouldn't have been as funny or interesting. In Toy Story, Woody becomes jealous of Buzz but when he realizes he is a fake,they become friends. Again Woody and Buzz would never have returned home if it wasn't for each other. I remember the main song in Toy Story is "you've got a friend in me". in The Incredibles, not one of the family could have beaten Syndrome and his army by them self instead they had to have distractions with the kids.
You can do anything even your dream
This is more noticeable in the perceived scary movies based on what it's about: Ratatouille and Monsters Inc. In Ratatouille, Remy wants to be a chef but can't because he is rat. But is able to when Linguini talks to him and realizes he can cook. At the end, (sorry if you haven't seen the movie yet) he becomes a chef when Anton Ego gives him a great review. As I have said before in Monsters Inc, Sulley likes kids and doesn't want to scare them but wants to joke with them and finally is able to when he jokes with the children instead of frightening them.
I always like the fact that Pixar makes a way for things that people hate and then make it so that they will get used to and grow accustomed to. such as Bugs, Monsters and now Rats. It's setting up for a new generation to not screech at rats or stomp at bugs and not scream at monsters.
Don't ever give up
In almost all these movies there is a brief moment when the protagonist is doubting himself. For example when Nemo get into the whales mouth or when Speedy gets lost to Radiator Springs or when Flik gets rejected by his crew. Quickly the characters get out of their doubts realize this will help me be better.
Being short is okay and being tall is not awkward
This isn't really a theme it's just something I have noticed in the films. The costume designer in The Incredibles is short and she is doing well for herself or Elastigirl is tall and she is able to handle herself. In Ratatouille, Remy is short compared to humans and but that's the same with all the rats. Linguini is taller at the beginning but then when we more used to him he seems shorter.
In conclusion, All of these Pixar movies have a lesson of friendship and be okay with yourself. I am glad that parents flock to the theaters with their kids when a pixar flick because it is spreading good messages.
Feel free to add on to this list in the comment section. such as more examples or themes you have notcied while watching these flicks. If you realized I didn't include much on "A Bugs Life" because I haven't seen it so if there is something obvious just write it down.