WHY did the writers have to go and ruin Walden's romance with Kate?!?

Their relationship was honest and real, and in my opinion the scene where he closed the door behind her and said, "My name is Walden Schmidt, I am a billionaire, and I love you," was the sweetest moment of the ENTIRE SERIES!

He had feelings for Kate for over a year, even though he did date other women after she left for China (which disproved his therapist's diagnosis that he fell in love with every woman he had sex with, btw), and then, just as he was about to propose , he falls for someone else?!

Ugh. 😣