for example, 電車で傘を忘れてしまった which translate to "i forgot my umbrella in the train" and it shows that the speaker didnt intend to do that and also shows negative emotion which is regret so my question as a foreign learner when i was scrolling on instagram, why is there sentences that shows positive emotions using the grammar Vてしまう? and i tend to find them in commercial posts, so i thought is it a commercial expression?