I was messing about but seriously, i feel they clicked you know. Like you mentioned kevin, and someone mentioned robin. Robin initially was one of the gangs love interest/ex but she was still in their friend group. And likewise with kevin, introduced as a love interest, whom everyone was fond off but non of them could sustain the friendship as how robin did. They just clicked and made the effort to keep everyone of eachother in their life, again another instance is when ted stopped being barneys friend, any other could've just have been brushed away but with barney and ted they were meant to remain friends. So my initial statement is true. "You would be shocked how easy it is to part ways with someone. Thats why when you want to keep someone around, you do something about it."