Just because you got along at work it does not mean that he is a friend to you. It easy to have a friendly relationship at work when you are in a team, even sharing some personalish things like kids, funny stories about a spouse, or standing up for an injustice in the work enviroment. But, that does not mean that you are friends or that he owes you anything.
When I put in my 2 week notice at my job, years back, many people were shocked and surprised because they all thought they knew me very well. In actuality, I shared only anecdotes of my life but people felt invested because I was a friendly person. []
I enjoyed the people at work but they weren't my friends, they were my coworkers with whom I had a good report. I did not or do not feel like I should have contacted them after leaving.
It is very possible that you felt more invested in him as a person, but he may have just been a friendly coworker.
You said he was your supervisor...
Perhaps all you two had was a work relationship. Work is over, so is the relationship.
It's probably the same thing, he was in a bad environment and is moving on. Also, you may be one of many people looking for an extra explanation... how many times would you want to rehash that if you were him?
A lot of folks confuse "friendly co-workers" with "friends."
If you and your coworker spent weekends at each others houses, went on vacations together, etc, I could see it unusual he didn't say goodbye.
If you were just co-workers, you are just cogs in the same machine.
I think maybe you thought you were friends while he thought differently.
All of the above, especially the bolded. Over the last few years, having a long-term chronic illness that limits my ability to work, I've picked up numerous short-term work at special events/seminars for extra quick cash. I've been able to network with lots of co-workers who refer these projects to me when they hear about them.
I've worked at some wonderful events

and also for some extremely horrible people & environments.

These "day jobs" are not my
avocation. They are just a way for me to make money to live
the rest of my life after work, which are my passions. Work has become just a means to an end.
I'm always very friendly, chatty, and good at what I do at work. (Which is how I get all my referrals for more work.) But when it comes to socializing after work with any of these co-workers, I am sooo not interested, though I may go to lunch with them
during work. That work may be part of their life, but it's not to me. It's just WORK. I've been very good at compartmentalizing them.
And at the end of most events, I just left without saying goodbye. Or did just a quick hug if coworkers I'm friendly with were nearby. When the work environment has been particularly bad, I can't get out of there fast enough, even though there may be a few people I DO want to say goodbye to and to tell them that
they are the only reason that made that project even palpable to come back to each day. But, if I can't find them easily, I do not linger to find them. It's not personal. I just can't stand the place and can't get on the subway fast enough to end that situation and move on, literally.
Recently, one event was so bad, I was counting down the days till it was over. The manager was brand new to managing and she was horribly disorganized and CLUELESS that she wasn't even good.

She even didn't show up one day and texted all of us what to do, although it was a job that required direct supervision over everyone.
At the end of the event, she planned this big picnic the following weekend for all the workers. To her, moving into management may be a big part of her life. To me, it was just another event, a bad one at that, and it was OVER. I didn't have to attend to keep up any happy work relationship appearance. I will actually turn down any project she ever supervises again.

So I never answered ANY of her group emails or texts, right up to the hour before the picnic.
Last week, I talked with 4 co-workers from that event, while we were on a new special event that I got them on. (Again, we don't socialize after work, just friendly emails to pass on work referrals.) I asked them if any of them attended the picnic. Not a single one attended, even though they made noises
during the project like they were interested. It seems more people than I realized, feel the same about compartmentalizing work from the rest of our lives. Now, I'm wondering how many people did show up.
