In general I like working on cars. However the following things can make it a lot less enjoyable:
1. Electronics and computers as they are difficult to work with and expensive to troubleshoot.
2. Rust on EVERYTHING underneath a vehicle from driving up in the Salt Belt.
3. Bees, wasps, and hornets.
4. The fact that cars today are essentially designed as "no user serviceable parts inside" appliances. There is often not a lot of room to work on these cars, many things are sealed "for life" (the life of the part, not the life of the car...) and some cars such as BMWs even lack dipsticks. You're not even supposed to check your own oil? What's next, tires with no valve stems so you have to replace the tires if they get low?