well maybe they thought they were in love, but it wasn't actually.
at that moment it may have seemed to them, that they're in love..n they're perfect for each other..but when u feel uve found someone better, ull obviously go for them. n sometimes people get bored of the same person, if they're not madly in love. So when they met an interesting person, they tended to fall for them. So its pretty reasonable..since they were never actually in love as they might have thought..so none of them is to blame here..
I know some guys are together because they are so comfortable with each other, they are afraid of trying something new. Six years is a long time and they probably got together in college or even earlier. I'm sure when they met new people and realized that it wasn't so bad and they could be attracted to other people they mutually decided to give each other space to venture out and try and live their lives. They probably still loved and cared for each other as most close friends do