Conditional love: I love you because of your merits.
Unconditional love: I love you without respect to conditions.
It’s interesting how unconditional love is not directly caused by conditions, but it’s not completely independent from conditions either. Eg I think you can intentionally cultivate unconditional love for someone, and you might choose to do that because of how good your life is with that person, which is conditional on their merits. And I think one way to cultivate unconditional love is to habitually feel appreciation for them, which will often be prompted by some little thing they said or did.
If “bitterness” is anger that forgot its cause, maybe unconditional love is appreciation that forgot its cause.