You can prevent your Queen's mood from changing during Breeding by applying a Mood Stabilizer. Mood Stabilizers can be created by redeeming Mood Stabilizer shards. View your Mood Stabilizer shards by visiting your inventory.

Why would I want to stabilize my Queen's mood?
When you match your Queen's mood to the moods of the Bees you send on missions, it generates significant stat boosts. To take advantage of these boosts, most players try to build their hive around the mood of their Queen by buying and/or trading for Bees that match her mood thus really want her mood to stay the same.