Donnie never dated anyone. He never did, was never super interested in romance. He just went on his own in life until he met you. Then he became a romantic. He loved you, despite showing it in different ways. This led to odd behaviors, such as prolonged, silent, unbroken eye contact when he needed something, or sometimes he'd forget very important dates, like your birthday, and completely avoid the blame later. Once he even just sniffed your hair and then act like nothing happened.

He also adored to show you off.

There's another love language and that's called 'I am fueled by love and suddenly gain confidence in everything I do due to it.' Donnie had that. The only people he didn't show you off to was to his brothers, but that's because he knew they would tease him incessantly. Even then, he would occasionally brag about your relationship with a proud smirk. He was still a huge dork, just a more confident one. But because he wants to praise you and appreciate you so much, he starts to compliment himself when he can't praise you enough.

One day, you two were getting some takeout at some fast food restaurant. You both waited in line for a bit while Donnie forced himself not to complain about the state of the tiles. He knew employees were paid minimum wage, but he hated the feel of sticky shoes.

You both ordered and waited for your name to be called. When they did, your order was... Wrong. Not by much, but enough. Donnie was furious. He stamped up to the register person. "EXcUSE ME, but my WONDERFUL, AMAZING, MAJESTIC partner whom I love dearly, quite CLEARLY requested no pickles. They are a QUEEN among men, and I am SO VERY lucky to have them in my life, so I suggest you remake her order." He snapped.