It's an informal expression. It means something like
"since you are so serious and everything like that"
"since you're so seeee-rious and grim and crabby and so on. . "
I would use this to subtly deprecate the other person's seriousness, and to imply that it is a pose (and maybe in other somewhat sarcastic uses), as well as more-or-less sincere uses.
> "Well, since you're so "busy" and all, I guess you won't have time for me!"
> "She was so "glamorous" and all that my boyfriend kept on staring at her."
> "Well, since you're so bored and all, how about cleaning out the garage?"
> "I guess I'll have to pay for lunch, seeing as how you're so broke and all."
> "If you're going to be so critical and all, I think I better just hang up now."
I suppose it is used to extend the description to a cluster of related words:
- Bored and all = bored and idle and restless (and all the ideas related to bored)
- critical and all = critical and negative and fault-finding etc
Student or Learner