I don't own anything!
Loving someone infinitely was a concept that Jade did not believe in. In her experience nobody could love someone other unconditionally. And she was sure that she herself might only mess up all relationships that were to come.
But then she met Beck. He was perfect. Not only did he look fantastic (long hair to play with; a full mouth perfect for kissing; dark eyes that could melt her heart) but he also was stubborn and understanding. Without these qualities their relationship might not have lasted longer than a few weeks. (Well, technically speaking, their relationship wouldn't have started at all without these traits.)
However, Jade was still sure that she might mess everything up. Beck was too perfect for that, it was sure that she would be the one breaking his heart. But Beck wasn't willing to let that happen, he wanted to show his girlfriend that a lasting relationship was possible. And that she was capable of loving someone a long time. So he needed to make her realise that she loved him, over and over again. Him just telling her wouldn't do, she had to feel it.
That was the only reason Beck still flirted with lots of girls. They didn't really interest him at all, the only girl he wanted to have loving him was his girlfriend. (Nobody compared to the hotness of her. The white complexion, the clear blue eyes, and the way she walked.) But making her jealous always worked. She would feel the need to be possessive of him, show the world that he was hers. And so she realised over and over again that she was still in love with this one boy.
At the time Tori Vega was starting at Hollywood Arts Jade had been depressed once again because another long-time relationship had ended and she once again doubted her and Beck could last. Her neighbours had split up after being together for 3 years. Beck and she had been dating for nearly 2 years and they were still in High School. (Everybody knew that couples from High School wouldn't last.)
For Beck this only meant he had to try harder to make her jealous and convince her of her love for him. This was why he kissed Tori on her first day. And man, did it work. Jade was over that evening at his RV and telling him (well, shouting it) to never do such chizz again because she loved him and he should be more considerate of her feelings. Beck being the good guy just let her rant and then kissed her, assuring he of his feelings again. But inwardly he was cheering, happy that once again his plan had worked.
When she broke up with him for everyone other than Beck it seemed randomly and especially Tori told him to just forget her. He couldn't do that because he knew why she had done it. (It wasn't because of Alyssa Vaughn. She had heard about a break up once again that had affected her: the grandparents she hadn't known about till now were happily married for 15 years and then suddenly they divorced.) Beck knew this time it was harder to get her back to loving him. She could go in two directions: 1) realising she really did love him and come back or 2) telling herself that relationships don't work and shutting herself off from all people.
Dealing with number 1 would be hard but manageable, however if she chose number 2 Beck would need a miracle to get her back. Luckily she did realise she missed him quite fast. But to really show her the extent of her feelings he had to let her make her work for the relationship. He couldn't just take her back after one day and her telling him: "I miss you." So he waited for a week, told Tori that he was happy about the break up and waited for his girlfriend to do something. When she finally came back she was sure that he loved her as much as she loved him.
For most of the people at Hollywood Arts the relationship between Beck and Jade seemed strange. They thought Beck was a womanizer but was too afraid of his girlfriend to end it and Jade was a possessive bitch that was tying Beck down against his will. Only their closest friends saw a sweet side of them, saw that there was love. But also they didn't know the true extent to the feelings the couple held for each other. And nobody other than Beck knew why he needed to flirt with other girls to make this relationship last.