This is the 2875th most frequent Japanese word.
ついつい
Unintentionally; subconsciously; out of habit.
The word 'ついつい' here is used to denote an unintentional or subconscious action. It expresses how the speaker, despite their intentions, ate the cake without realizing their own automatic behavior.
ダイエット中なのに、ついついケーキを食べてしまった。
Despite being on a diet, I unintentionally ended up eating cake.
In this sentence, 'ついつい' describes an involuntary emotional response that occurs naturally upon witnessing the smile.
彼の笑顔を見ると、ついつい安心してしまう。
When I see his smile, I can't help but feel at ease.
Here, 'ついつい' refers to the unintended consequence of time passing subconsciously while being engrossed in the activity.