One reason is the death of either of the husband or a wife.
1 Corinthians 7:39 A wife is bound by law as long as her husband lives; but if her husband dies, she is at liberty to be married to whom she wishes, only in the Lord.
The second reason is marital unfaithfulness.
Matthew 19:9 And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery.”
Not all may accept this though many are divorced and many are still in a relationship where either the other partner or both are committing infidelity.
Jesus taught us to forgive those who sins against us.
Matthew 4:12 and forgive us our sins, as we have forgiven those who sin against us.
He wants us to forgive those who sins against us not once or seven times but seventy times seven or 490 times.
Matthew 18:21-22 Then Peter came to Him and said, “Lord, how often shall my brother sin against me, and I forgive him? Up to seven times?” Jesus said to him, “I do not say to you, up to seven times, but up to seventy times seven.
At the same time He warned us not to make ourselves at home among those who indulges in sexual sin or the sexually promiscuous.
1 Corinthians 5:11 But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner—not even to eat with such a person.
Be that as it may, there must be true repentance from the offender, prior to forgiveness, that it won't happen again. The marital unfaithfulness must not be committed even once, but when committed the forgiveness must be given only once, otherwise, You are allowing him/her to take the infidelity training hoping that he will graduate soon, after repetitive offenses, to finally know the difference between right and wrong.
The question lies on how far can you tolerate your partner though one offense is permitted for a divorce in God's law.