When I DM D&D, combat should be interesting, challenging, relevant and drive the story forward without being tedious and boring.
I want my players to jump into the right fights, for the right reasons, without too much hesitation.
I want my players to avoid unnecessary fights and fear the dangers of combat.
Flight and avoiding combat entirely is sometimes a necessary choice for the players to survive.
As a DM I need to be consistent and fair. Enemies must not be much more dangerous than they appear, at least not without a way out. Enemies that appear to be more powerful than the players are deadly enemies and will capture or kill the player characters.
If my players are actively seeking clues to whether they should engage in combat or not, I shall give them clear and helpful information.
Rests: I will usually apply the gritty rules for rests: 1 night for a short rest, 1 week for a long rest. I will occationally grant faster rests.
Background
I like the fast resting rules (1 night for a long rest, 1 hour for a short rest) because it keeps the story going forward.
However, I have now learnt that the resting rules can be (ab)used to engaging into unnecessary combat not caring too much about injuries, rather than finding non-combat solutions. I have also learnt that player characters in D&D 5e are quite powerful and having the characters always rested means I have to make opponents consistently more powerful and combat takes longer time.
I expect a typical session to take 4-5 hours, the characters start rested and there should be 3-5 combats. About 1-2 rests should often be enough.
The purpose of this is to make D&D combat interesting, challenging, relevant and drive the story forward without being tedious and boring.