By commonality of aspiration, commonality of thought, commonality of heart, commonality of life and physical existence.
Naturally this commonality cannot be imposed, either it is there due to affinity at different levels or else it develops with time if there is love because one tends to grow into the likeness of the person we love.
Of course a common aspiration is the most important. Physical togetherness leads to attachment but if the egos rub too much then there is a tendency for love to pass away.
Affectionately,
Alok Da.
