I don't have a brain tumour, but I have had, and am still having, an experience which has made me think seriously about my mortality.
I'm 31. About three years ago I started having three/four day long bouts of total delirium, roasting fever, extreme nausea, can't even keep sips of water down when it happens - by day three or four, I'm usually too weak to move, totally dehydrated.
Two years on, my GP is still going "clearly your hygiene is terrible and you're giving yourself food poisoning", and it had got to the point where I'd be lying there, lapsing in and out of consciousness, feeling my heart stop. And start. Then stop. Then I'd turn blue. Then it'd start again. Then I'd turn back to a nice healthy jaundiced yellow.
Took myself to the far side of the planet, got diagnosed with gallstones. Not just good old fashioned gallstones, but gallstones that make an imaging tech go pale and run out of the room to bring in several other people to hemm and haww before looking at me and going "Sir, you're extremely ill.". Turns out my gall bladder ruptured years ago, due to a large collection of stones, and occasionally oozes bile and bowel contents into my innards. It has eroded my vagus nerve (hence the cardiac symptoms), damaged my liver, and I'm having surgery at some point hopefully soon to have the damned thing (gall bladder) outright removed. I can't drink any more, and my other half gives me no end of shit for smoking weed in my spare time, as it's the one thing that makes the f'ing thing stop hurting - or at least makes me not care that it hurts. I've tried various legal pharmaceuticals, but they all either make me more nauseous, or make me incredibly tired - or I can't take them because they're liver metabolised. I digress.
Anyway.
The long and the short is that knowing what's wrong with me has helped, as when I didn't know it was utterly terrifying, and while it's now still terrifying, it's a known terror, rather than my darkest fears made manifest.
I could drop dead with only a few days notice from this, and my next attack might be my last. Every time I've had a major attack, I haven't just felt like I've been dying - I have been dying - and have been fortuitous enough not to. For now.
This led to the realisation that I could drop dead at any point, regardless of this. I mean, we're all ultimately aware of our own mortality, but until you're reminded of it every day, by perhaps an awkward "I have a stick wedged under my ribs and looking at things makes me feel sick" sensation, you never really appreciate it.
Mortality is the ticking timer that keeps you alive.
If the mortality of others can remind us of the importance of living well, then even if just in that way alone, they persist in doing good for we the living even once they're gone.