Why did Han seem unhappy that Rey bypassed the compressor?
They both agreed that it was a bad addition to the Falcon.
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The film script indicates that he wasn't unhappy, just confused.
HAN: This hyperdrive blows there's gonna be pieces of us in three different systems.
Han at the controls when all the alarms STOP. Rey, satisfied, sits in the co-pilot seat. Han is confused.
HAN (CONT'D): What'd you do?
REY: I by-passed the compressor.
He looks at her. A little, appreciative laugh. Han exits, walking past BB-8.
When I saw that scene, I figured it was two things.
One was that a young thing was moving faster than him and matching him for skill with his ship, his baby. She basically had mastered the thing he spent a long time trying to recover. The Falcon was his thing and she handled it as well as him.
The other is that he was feeling pretty old; few people like having children (probably from his view) exceeding their skill.
A third is that maybe he was wishing it was Ben sitting there, being all excited about solving the problem.
There's an answer here that references the script. However, the script doesn't address Harrison Ford's protrayal of the character. I'll say this:
Han wasn't unhappy - and he wasn't just confused - he was impressed.
In this movie, Han's archetype has changed slightly; he's still a smuggler, but he's feeling his years. Partly due to the character's age, and partly due to Harrison Ford's Cool Old Guy status, Han comes across as a Retired Badass. Rey continues to impress Han with her feats (such as fixing the hyperdrive issue) to the point that Han offers her a position aboard the Falcon (albeit with grudging respect, playing it cool up to the very end, the sneaky dog).
Essentially, Han's reaction is just that he's playing it cool. That's his meal ticket and it's an essential part of the character.
Here's the clip: