Good friends can talk about practically anything without worry of having hurt each other's feelings. The mane six are those kind of good friends. When they get together, they can talk about nearly everything and anything.
But girls can be very, very weird when they all get together for a session of girl talk. Spike knows this and wisely stays away from them when they get like this.
Ponyville's resident human, however, despite warning from Spike, decides to walk in on one of their sessions as it's going on.