In Season 3, after Allison Cameron breaks up with Robert Chase for getting too emotionally involved in the no-strings-attached sexual relationship they were having, Chase started I love you Tuesdays to remind her every week that he was still in love with her.
Cameron breaks it off after the events in Airborne. Chase makes it clear to her that he believes Cameron can't possibly be callous enough not to have an emotional attachment to him and won't make it easy for her.
In Act Your Age, Cameron is still angry with Chase, who points out she can't both break off the relationship and be angry with him. However, they come to an accommodation and, later in the episode, he gives her flowers.
In Family, Chase tells Cameron he likes her while they are performing tests in the pathology lab.
In The Jerk, Chase reminds Cameron that he likes her right after she accuses him of sabotaging Foreman's job interview.
After Chase is fired in Human Error, Cameron meets him in a bar after work and he admits the whole idea was stupid. They part, but later that evening, Cameron comes by his apartment to say he wants to hear him say it again even though it's only Monday. Their relationship starts at that point.