The American Heritage Dictionary defines soulmate as: "One of two persons compatible with each other in disposition, point of view, or sensitivity." When we are with a person who seems to understand and support us, we may feel loved, content, peaceful, or complete. It is certainly wonderful to feel understood.
However, understanding another person varies with time and circumstances. As an adolescent and twenty-something, one's emotions, hopes, and ideas vary greatly. A person is exploring and defining his identity.
In adulthood the personality is more fixed; yet, it is still growing as the self engages in diverse experiences: parenting babies or teens, changing careers, aging, suffering with cancer, developing new interests, etc. Life experiences change us. Consequently, one's soulmate, or a person's needs in a mate, change over time. Does this mean we should find a new partner as our own soul changes? No.
Scripturally, marriage is a lifetime commitment. Jesus said of a married couple that "they are no longer two, but one. Therefore what God has joined together, let man not separate." (Matt. 19:6) Once the serious choice of our lifemate is made, it becomes our responsibility to nurture the relationship and to honor our vows. Feelings in marriage always pass through the initial infatuation stage. God knows this. Yet His arrangement is that we maintain our commitments and go on to develop a mature love for each other.