You can ignore triggers
Both Someone_Evil and Willem Renzema quote relevant rules texts in their answers and are correct in their interpretations.
For me, the clearest statement that reactions are optional comes from the rules on readied actions (emphasis mine):
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.
I like this passage because it explicitly establishes that one can refuse to take a reaction, rather than having that be implicit through the use of 'can' and 'allow' when describing the positive choice to take a reaction as the rules cited in the other answers do. By preferring it, though, I am assuming that the rules for taking reactions permitted by readied actions are no different from those of any other reaction. I don't know why they wouldn't be, but it is a fair objection if one doesn't want to assume that.
The passage establishes that each time you have the ability to take a reaction due to a trigger, you can decide to do so, or not. This is a strategic decision since using your reaction for that particular trigger then removes the possibility of reacting until your next turn, while not using it preserves your ability to respond with a reaction to any other trigger that occurs until then.
Your intuition that forced reactions would drain spell slots is correct. Consider also any melee character that closed the distance to an arcane caster - if one were required to take any available reaction, it would be trivial for you to move into the caster's reach, then move out again to force them to make an opportunity attack (with what? a dagger?) and then move back in again, confident that they could no longer use their shield spell to protect against your attack. This would considerably reduce the power of the shield spell, as well as many other abilities that either force your opponents to take reactions or remove their ability to do so.