The way I interpreted it, they’re both happy in the relationship, but both are telling eachother (but really themselves) that they aren’t going to commit, because they’re both afraid of becoming invested and subsequently hurt when it doesn’t pan out (likely due to bad past experiences). But they both act like this because they’re afraid of the other abandoning them. Eventually they’re together long enough that it has since become clear that neither of them are leaving and they’re both happy, so the last panel is them finally addressing that fact.

It’s a joke because 25 years is an absurd amount of time to come to that realization. In reality this couple would have figured this out after a few years. It’s exaggeration.

It’s “cute” because they’re both good, loyal people who love each other, but they were both on the defensive for a long time, which is an immature “toxic” behavior, but because they were both on the exact same page, they have grown out of that toxic behavior together.

The guy does kinda comes across like maybe he’s not sharing the same behavior because he’s a bit more nonchalant about it, but I think that’s the general idea.

Calm down Freud, it's just a gentle comic!