Bees can be irritated for other reasons than having mites.
From Carl's Sims 4 Guide:
While it isn't really the case, bees kind of have a social need. They gradually get angrier and angrier if you do not interact with them for a time. At first they're irritated, which increases the chance they sting you, but then grow enraged and a sting is near-certain. By interacting with your bees daily, you can keep that friendship meter up, and give them a sort of 'recently bonded' moodlet that keeps their mood Calm. Bees will sting you while calm sometimes but not nearly as often as they will when irritated or enraged. Getting your bees off enraged is a matter of using bond a few times, to get them calmed down. You'll have to endure the stings! This is the penalty for letting them get so upset. Sims who disturb your bees will also raise their anger (but get a playful moodlet in the process).
I've also read somewhere that very hot or cold weather can upset them if they are not kept in a greenhouse.
