Over the past 2 days, I’ve listened to every Pixar soundtrack and noticed something strange. In every track on the separate CD’s, they seem to repeat bars over and over again. i.e, in Up, it’s the Married Life song repeated. Has anyone else noticed this?
I think it is not uncommon to have certain themes repeated in some way throughout a film score.
For example, in the first Lord of the Rings film you have the Fellowship theme that runs like a thread through the entire score. It changes multiple times (like “Married Life” does), but you always recognize it.
So this is nothing particular to Pixar scores.
Yep, writing 1,5 hour worth of completely original score is undoable. 
And having a specific theme for certain situations with several varations of it works more than having some random song playing. For example, there’s a theme for danger, a theme for character A, a theme for character B, a theme for the love interest between A and B, a main theme, etc.
Actually, I can’t recall a movie that hasn’t done that with it’s soundtrack actually.
In my opinion it keeps the same feel for the same movie. I think it’s a good thing.
Repeated melodies help emphasize certain themes and scenes in a movie. But that’s just my opnion.
Example: “Across the Stars” (Love Theme) in Attack of the Clones is always played when Anakin and Padmé are together. Star Wars fangirling alert!
Or the Imperial March when any Empire thing is shown
Yes.
It’s called leitmotif. Of course the best user of this is John Williams, but almost every soundtrack has at least one. It serves narrative purposes, even if almost no one notice it. Personally, I’m a soundtrack lover.
This is up to debate, but fortunately I agree with you. 
Film score business has the best compositors in the world. They are all very good, but Williams is the best (he’s the most Oscar nominated person alive) and his leitmotif use is legendary (two of two examples on this board are from his work).
In regard to Pixar, my favorite leitmotif user is Michael Giacchino. That man is a genius and will have a lot of success in the future.
I personnally have to say that I prefer Hans Zimmer over Williams, though… but when it comes to ‘leitmotif’, you’re right about Williams.
On the Up Blu-Ray is a featurette called “Composing for Characters,” where Michael Giacchino talks specifically about using motifs for characters, how “Ellie’s Theme” keeps her present through the film, etc. I love how he points out that, when Muntz first invites them onboard the Spirit of Adventure, his theme is very lush and romantic, because it’s Carl’s idealized view of his childhood hero - then, when Muntz starts obsessing about the Bird, his theme becomes distorted and discordant and the audience realizes, “Oh, that’s not good…” I love MG’s score for Up, and was so glad it won him the Oscar!
Funnily enough, I’ll be studying leitmotifs/motifs in the near future for my Music course. From what I know already, leitmotifs are in itself a melodical device that accentuates an idea (which what everyone is saying). Just remember that movies are not just visual…
Oh, and for the record, I’m sure to be using the soundtracks of The Incredibles, Ratatouille, and Up to back up my knowledge as examples during the course. 
This is common in film soundtracks. Pete Docter said that the repetition of the “Married Life” melody is supposed to give you a sense that Ellie is with Carl throughout the entire movie. I think it’s a very artistic way to convey a message and to arouse emotions in movies. 