The connection between Cherilee and Big McIntosh is easily seen while the antics of the Cutie Mark Crusaders were trying to get them together. But there is more to the story then them just being acquaintances. Hints can be seen in how they first react when alone together and the slight awkwardness in Cherilee's attempt at small talk.

In order to better understand the relationship between these two - why the Cutie Mark Crusaders thought they were a good match, and why they are undoubtably not the greatest of matches - derives from their younger days.

Cherilee and McIntosh were special ponies with each other at one point, but as most young foal relationships the two drifted apart in their long term goals and parted ways.

And also Thunderlane for two reasons. One - rule of three. Two - more interactions in general.