Trying to read a proof in depth (say, because I was preparing a blog post on it), the reader may go through various feelings in sequence.
1. Stupidity; How come I don't understand it at all? Am I becoming stupid?
2. Anger and frustration; how come the authors leave some definitions ambiguous, and why did the referee not send the paper back to ask for another round of polishing?
3. An idea: let me just look for slides of a talk on the subject! Slides usually put much more emphasis on intuition and might help me understand what the authors meant to say.
4. Obstination; I can do it. There are no slides available, but I'm interested in the result and just need to put my mind to it. Go through examples systematically, try out the various interpretations until one fits and the proof goes through.
5. Suspicion. Is that result really correct? None of my interpretations seem to work.
6. Discouragement; I can't do it. I'll have to look up references of prior work and see which ideas are present in there and whether they are explained any better. This is turning into a major time sink.
As an intellectual activity, reading technical text is really not much different from doing research.