There is always a chance of failed interactions. Remember, emotions play a big role. So if there any tense moodlets, even hidden behind a flirty moodlet or confident, what have you, it can still bring the chance for a negative interaction up. The other thing to keep in mind is that confess attraction is one with higher value, though I thought that as long as you had first kiss, it shouldn't be bad. But if there was some time in between the first kiss and when he confessed attraction, it might have been enough for the pink bar to just dip down just a little. I'm more likely to go with a tense or uncomfortable moodlet hiding behind a good moodlet though as the cause.