Your angelfish are kissing because they are either mating or fighting. Angelfish lock their lips as a part of their mating interactions right before looking for a spawning site. They can also lock lips to show dominance.
Distinguishing between the two behaviors can be difficult, especially for novice fishkeepers. First, try determining the sex of the fish. If they’re of the same sex, then it’s likely they’re fighting for dominance. Additionally, see how they behave after locking lips. If they swim together side by side, they’re mating, but if they start chasing each other, they’re fighting.