Unlike his counterpart, Phineas knew all about the concept of love. It was when a guy and a girl experience feelings that indicate they want to be more than just friends. But despite the knowledge, Phineas himself never experienced love. That is, until the day Doofenshmirtz was defeated.
It all started at Macy's. Phineas and Ferb were helping Isabella get a new look, but she chose an ensemble herself. Shortly afterwards, Ferb kissed Isabella and she enjoyed it. It wasn't until the next day that Phineas realized he was jealous of Ferb for dating Isabella. So in order to get Isabella to notice him, Phineas decided to take an interest in inventing.
Sadly, no matter how hard Phineas tried, he couldn't seem to get his inventions to function properly. He was about to give up when he met a young girl. She was Vanessa Wilson, Isabella's cousin. Phineas got along with Vanessa very quickly, although he refused to let go of his crush on Isabella.
But the next day, something strange happened. Phineas was trying to get rid of the paint that was on him after Candace defeated him at the extreme paintball arena when Vanessa came over to visit. She was able to help Phineas with his inventions by pointing out a small error on his blueprints. So after using his car-free carwash to clean himself from the paint, Phineas was convinced into telling Vanessa his dream of a perfect family. He was in tears afterwards, so Vanessa felt bad for him and she tried to cheer him up.
Then, Vanessa reached up to Phineas' face. He knew she wanted to take off his sunglasses, but Phineas couldn't stand telling her 'no'. He decided to let her see his eyes.
"I never thought you would have such beautiful dark blue eyes."
Phineas wondered why Vanessa would say that. His eyes weren't that special. But somehow, that comment made him feel happy. He knew Isabella wouldn't say things like that.
That's when he realized what was going on with him. He wanted Vanessa to give him more compliments that made him feel happy. He decided to let go of his crush on Isabella and move on with Vanessa.
Phineas looked at Vanessa as he smiled, seeing how pretty she looked in the sunlight. That's when he leaned forward and kissed her, wanting to see if she was his dream girl.
And sure enough, Phineas was right. He heard fireworks going off in his head as he placed his hands on Vanessa's waist. She kissed back almost instantly, putting her own hands on the back of his neck.
Phineas might have been crushing on Isabella, but in the end, he fell in love with her cousin.