Especially when it comes to your partner, expression of your real feelings is of great importance. When you really love someone and have true affection for him/her, reminding him/her sometimes will help you built a relationship based on trust.
We all suffer from insecurities, and though you may know your partner well, you cannot always know what’s in his head. So in this case, being sentimental and sensitive will be more than beneficial for you both.
Of course, the same thing goes for the occasion when you have to let someone know that his feelings of fondness and devotion are not mutual (or no longer mutual). It might be heartbreaking for you but holding on to an unsatisfying affair, will in the end be more painful for both of you.
Moreover, in all types of relationships, the relief you get when you sincerely tell someone how you feel is undoubtedly more worth than the feeling of regret you may have for not saying how you really feel sometimes.
And not to mention, the anxiety and discomfort that comes with disclosing your feelings. That feeling of peace when you know you spoke the truth will surely make you feel happy with yourself, regardless of the reaction of the others.