I noticed that the front passenger side pads were thinning real quick about a year ago. So I go ahead and check if the caliper is stuck and it was. So then I get myself a rebuilt caliper and swap it in, changing the rotor and the lines as well. Now, the same caliper on the same side is stuck again. The drivers' side caliper is fine, normal pad wear. What can be the cause of this?

maybe try out a new caliper? it shouldn't be that expensive right?

no, its not too expensive. the problem is though, that the i think there's a problem with the system on the passenger side because it's the only one that consistently gets stuck.

Flush all the brake system?

You say the caliper is stuck again. What happens when you try to compress the piston back in with C-clamp and block of wood? Last time you repaired did you replace or re-grease the caliper pins ie. is the caliper floating correctly?