The Rule of St. Benedict: Chapter 53, part 2
Benedictine monasteries functioned as a hostel system within early Medieval Europe, with a constant flow of guests from all walks of life, and the instructions here about a separate kitchen and quarters for guests enabled the monastery to still be a monastery despite the traffic. The monastic atmosphere was to be preserved in part by following the instructions at the end of Chapter 53 regarding the interactions of monks with guests. Monks are to be humble and kind to guests, but not necessarily nice.
Niceness, a perceived affect according to social norms, is a poor substitute for kindness, which is a genuine concern for the well-being of the other. It is easy for some of us to confuse smiles and pleasant tones of voice with the things that make for true kindness and hospitality. If we continue in this confusion for long enough, we become unable to distinguish between kindness and unkindness, hospitality and inhospitality.
My experience growing up as a person who was generally considered to be nice is that niceness is entirely oriented outwardly. It is something one measures by what bounces back from the exterior of other people. My developing ego was very successful at getting what it needed from this external exchange, and I have come to realize that being nice usually serves to keep me self-absorbed--caught up in my own self-interest. People who are regarded as reserved or distant, on the other hand, may exhibit self-absorption differently, but this disposition can be just as self-interested as the other.
It is possible for one person to be outgoing and nice without being self-absorbed, and it's possible for another person to be quiet and reserved without being self-absorbed. What matters is our ability to perceive clearly our own motives and to develop a sense of ourselves that is derived from our inner and true identity, our life hidden with Christ in God. When we act from that place, we are able to be truly kind.