It used to be that working on a dynamic team (with member of different age) would be a big source of motivation. But nowadays with remote working and teams spread all over the world (and managers and business units having different agenda sometimes), this seems to fade away, hard to find anyway.
What is left? Somehow I have grown to hate an unfinished job -- a failed, or a late, or simply a clunky product. I can't help but think that I've put in it days of my life that I count much more thoroughly now than I used to.
Another source -- I hate to fail people who puts trust in me, and they tend to do this more now than when I was younger.
Funny enough, people's managers in companies I have been don't seem to understand these motivational factors. In yearly career-development meetings, I absolutely had to provide a "proof of motivation" by coming up with a "career path" leading somewhere.
Genuine enthusiasm is what I think a lot of younger people have over older people. After getting enough experience, I just really can't get very excited over the whole mobile craze, or IoT, or yet another f'ing scripting language, or automation, or what have you. I do admit I got excited about VR for a few months, but that wore off fast, and would probably have worn off even faster if I had worked in VR and was immersed in it 12 hours a day, every day for years on end.
Some people can somehow keep their enthusiasm for tech their whole lives, and it never gets old for them. I envy those people, but I have many other interests, and my interests tend to change relatively frequently, and I just can't keep a single-minded focus on just tech (or anything else) for very long. That's why working in tech is just a job for me. If I wasn't paid, and paid very well for it, I'd just keep it as a hobby.
I don't think it's just tech for me either. I think I'd feel about any profession the exact same way. After a while, I just get bored and want to move on to something else, something completely new and different.