Starting Items can and should define your playstyle.
Choosing Items will have to do with:
-Your matchups
-Your and your opponent’s summoner spells
-Your goals with the laning phase.
The more aggressive the start (i.e. Dorans) the more your approach should be focused on ruthlessness and domination.
The more sustain focused (boots, cloth, flask) the more you should be expecting to take hits and be able to patiently withstand them.
Early game focused champions will tend to go for more aggressive starts, going for a more sustained focused item choice might not grant them the resistance to be able to go all.in on the enemy.
Starting item choice also has to do with what you expect your opponet will choose.
Champions that are “poke heavy” might pick a flask, even though they only need it for mana.
In order to counter such situation you will have to pick a sustained focused starting combo.
This WILL force you to avoid all-ins but it will reward you if you are patient enough.
Throughout the game, item decisions will determine how your matchup evolves.
Most matchups are not defined by item choice, they can just be “tampered” or snowballed by them.
Beside scaling, whenver you are in advantage you should buy defensive stats, up to a point that is defined by how much your champion needs damage (for example a riven should build damage pretty much always).
The reason for this stands in the fact that being ahead, will grant you possibly some base stas based on level advantage, and the gold invested in tankyness will basically zero out the chances of a comeback from your direct opponent.
Building tankyness is useful also when your matchup is about to turn on you, for example when a lategame champion is about to hit their powerspike.
You need to be as tanky as you need, in order to deal as much damage as you can.
Building pure damage on melee champions isn’t something I cringe upon, yet it is not an effective choice because you are basically reducing your chances of variating your play.
If you go full damage you limit your course of action to: “Have I killed them?” Yes or No.
Tankyness is also utility as gaining cooldown reduction along with stats will allow you to soak damage and cc your opponent for longer.
About tanking damage.
In an ideal world, the other team will ignore the tank and all focus a single target.
That almost never occurs.
A decent compromise consit of focusing the tank all toghether to keep them away from the carries.
That’s basically the tank’s role, possibly to kill the enemy carries, generally to soak as much damage as possible.
The first pre-requisite is your own carry’s safety.
Then you can think about killing your opponents.