There are basically two important factors to fighting.
'#1: Was someone caught, and are they absolutely going to die?
If someone gets caught out, and there's all of about 0 chances of their survival, you need to know how to not throw yourself away. I see people - fairly often - try to help an absolutely doomed team mate and die themselves. Generally speaking you're fine to throw some long ranged CC to attempt to peel and give them an opening, but do not get involved beyond that.
'#2: On the flip side, was it an intentionally picked fight?
I'm a jungle main. I'm a jungle main who prefers tanks (Zac, Sion) and fighters / bruisers (Olaf, Rek'sai, Trundle).
Of the champions I play, I've a very strong grasp of most of them. I'm aware of how much damage I can take and dish out, and given I've played for years I've a pretty strong read on how much damage most champs will deal to me (except Irelia, qq).
I will, at times, pick fights which look rather sketchy. Dueling a full health Xin at level 2, while I'm at 75% on Olaf at level 2, looks like a fight I'm going to lose; but it's not. The way the champions and their strengths play out, that's a fight Olaf wins 100% of the time baring outside factors. Likewise, picking a fight as 25% health Sion while my shield is up but the enemy mobility is down.
I'd say I can gauge when myself and my laner(s) will win a fight.. probably 90% of the team, at least in the early and mid game. But a lot of these fights look close, a lot of them look like baits, and a lot of the time my allies won't help me / cut and run too early, and I just end up dying for free / in a 1 for 1 trade.
To try and put this more succinctly:
Part of improving in league is pushing yourself into uncomfortable territory and learning how to navigate it. Playing safe is a skill all its own but often times - especially in normals - it's worth it to 'risk it' to get a better grasp of champions and situations. Fights which look doomed to fail can be easily won, and damning picks turned entirely around, with practice and confidence.
Don't feed and throw your life away, but push yourself to be a better player and pull off more impressive things. Also understand when the chips and down and you have to fight; when it's worth it to fight, and potentially die, in order to help your team get through.