I like to think that the reason why Loki went from showing his emotion freely to not showing any of it at all is because he’s afraid to get hurt again.
We have a tendency to mask our pain with a sense of bravado…emphasized by Loki’s ‘couldn’t care less’ attitude. For all we know after Jane slapped him across the face he could have been genuinely surprised that a mortal, a member of a race who feared him not that long ago possessed the audacity to do that.
Do we see that surprise?
No. We see a sense of amusement. Concealing any possible true feeling. Though you’ll see that the smile doesn’t reach all the way up to his eyes. It’s a masking smile.
Loki has a tendency to do that…After all the biggest lies we tell are the ones we tell ourselves.