wear one's heart on one's sleeve
Also, pin one's heart on one's sleeve. Openly show one's feelings, especially amorous ones. For example, You can't help but see how he feels about her; he wears his heart on his sleeve. This expression alludes to the former custom of tying a woman's favor to her lover's sleeve, thereby announcing their attachment. Shakespeare had it in Othello (1:1): "But I will wear my heart upon my sleeve for daws to peck at."