I think is that “a sense of vocation” does figure very highly in a lot of people’s motivations?
Once you get the urge to do something, you want to follow it through.
I’m not saying everyone has the urge to “do medicine” but I think that most doctors couldn’t think of doing anything else.
Arguably, I’d be a lot “better off” if I’d stayed partnered to my well off ex (happy with no kids) and worked part time as a receptionist somewhere posh in between buying things for the house using my ex’s “high six figure” salary and planning our next holiday.
Maybe write a shit blog so I could say how creative I was.
I’m now a single, mature STEM student (not medicine) . Even choosing masters courses , I’m going to aim for the one which costs more but will give me a better lead into the kind of research I want to do.
I want to contribute to solving stuff at the highest level I personally can manage (which I’m sure won’t be headline grabbing or have a huge amount of social status, but it’s what I want to do)
I actually don’t care if im sometimes a bit in my overdraft and live in a one bedroom flat on the shite end of town?
To save for my masters, I have to work part time in a supermarket. I genuinely don’t care if “Dave” the deputy manager at the supermarket drives an Audi at 23 and earns three times more than I’d get as a PHD stipend.
I’m not Dave. Dave isn’t me. Our motivations are different. Also I can’t drive 
I wouldn’t say I’m hugely altruistic or even unusual? I’ll need to think about stashing money away for retirement at some point, but I’ll mindfully and practically address that when the time comes.
If someone told me that I could get (say) a business job on £250000 a year tomorrow but NOT ever doing the research I want, I’d say no.
“The heart wants what the heart wants”