Why is snooping Vilified?

I’ve seen posts where girls feel bad for snooping even after they did find their SO wasn’t being faithful or Inappropriate in whatever way. I was in a situation where I didn’t feel my so was being honest but I also felt maybe I’m going crazy just needed proof. I did snoop and found out he was still contacting someone he had a thing for. We have worked through this and are in a much better space. But I always wonder because even back then I felt bad for invading his privacy. But now I don’t feel bad at all because I think I never would have known that if I didn’t snoop and I would actually feel that I’m just crazy insecure when my gut was telling me something is off.