If you feel happy about something and you express it, you express gratitude right ? And if you feel bad about something, you should also talk about it with the one you intend to build a life with. Why ?
Easy - Primo, because your partner is the only person who is or will be sharing every second of your life and not only positive ones but also bad ones. Secundo, because if you don’t share what makes you feel bad, you will be acting nervously, or irritatingly, or any way that will affect your communication and your daily life with that person. But, by sharing, communicating, expressing your feelings, whether good or bad, your mind as well as your heart will be relieved and you will boost confidence and trust between both of you.