I am going to try to be clear and short ;)
I had a job interview 10 months ago with a big company. I didn't have the job.
Fastforward to now. One of the people interviewing me added me on Linkedin. I accepted but I don't really know what to think.
He basically has a good job, in one of the biggest company in the country (world?) and I really don't. I am a little bit surprised he connected on Linkedin.
Obviously, he might just connect with everybody working in our field and add me without even remembering we had a job interview
so long ago.
Should I try to send message? And if so, saying what? "Hey dude, Wasup? Finally realising you made a huge mistake not giving me the job?" 
I guess if he wants to tell me something, he will find his way to the message button. But even if he does add everybody inour field, should I give use this opportunity to send a message, and then again to say what?
Oh, dear, I am overthinking this, am I not?