As you walk in the forest, you notice a group of gnomes, tightly packed together; your turn Mr Sorcerer. "I cast Firebolt" "You mean Fireball?" "Nah, saving it for later". Ten rounds later and with 10hp left the battle is over.
It doesn't matter if it's the first battle or one after 2 hours (and no rests) of playing; burning spell slots feels risky, leaves you powerless, and you're already thinking "what if I need this later". It's not really a player-specific issue; I feel the same most of the time, and, role-playing aside, it's really hard to stop hoarding spell slots.
While there isn't a lot to be done as a player (besides picking a warlock over a wizard), there must be a way to resolve this as a GM. Some thoughts:
Warlock on steroids: instead of starting with, say 10 slots, you start with 5 and after each encounter you have a 50% chance to restore a used spell slot. Pros: good motivation; cons: hard to balance
Divination spells: the caster can use divinations to determine when the "hour of greatest need" will be. Pros: makes sense in-universe, casters would have developed this sort of thing - also feels somewhat balanced; Cons: sounds like a lame minigame: burn a spellslot and I'll tell you when to go nova
Narrative clues: "you see a big tall figure with skull-decorated armour and a huge dragon as a pet". Pros: seamless (?); Cons: might be too subtle or over the top
Bit surprised this hasn't been asked before tbh (but maybe my investigation score is abysmal).