Mental solitude means first and foremost not to be dependent on others for approval, for companionable talk, for a relationship. It doesn't mean that one becomes unfriendly towards others, just that one is mentally independent. If another person is kind to us, well and good. If that isn't the case, that's fine too, and makes no difference. The horn of a rhinoceros is straight and solid and so strong that we can't bend it.
Mental solitude cuts out idle chatter, which is detrimental to spiritual growth.
Talking about nothing at all, just letting off steam. When we let the steam go from a pot, we can't cook the food. Our practice can be likened to putting the heat on oneself. If we let off steam again and again, that inner process is stopped. It's much better to let the steam accumulate and find out what is cooking.