You don't need a "reason" to divorce per se. You or your soon-to-be ex-spouse simply needs to want to get a divorce. That being said, there are many people who make the decision to divorce after years of dealing with the following less-than-comfortable symptoms of a failing and toxic marriage.
If your marriage is exhibiting any of the following signs, maybe you should consider whether your life could be improved by going solo:
You're unhappy: If you're not happy, and you've been that way for many years, you might want to stop and think about why. The unhappiness you feel could be sourced from a variety of reasons, but if you're certain that it relates to relationship strife -- and if you feel like your life isn't peaceful because of conflict in your marriage that never seems to get resolved -- it could be time to call it quits.
Your interactions are not positive and you're avoiding your partner: The interactions you have with your partner should be positive overall. And you should want to spend time with him or her. If you find yourself avoiding conflict and difficulty by avoiding your spouse, this is an indicator that divorce might be the best option you have.
Your friends and family tell you to call it quits and they won't let up: While there are the rare occasions of a successful "Romeo and Juliette" scenario, chances are that your closest friends and family members do know what's best for you. If you're not certain inside yourself that this person is healthy for you, and if you have countless people whispering in your ear to end the relationship, perhaps these people are right.
Your instincts say "run": What's the deal? Why is that voice in your head saying to run and don't look back? There comes a time when rational thinking has its limits. The fight-or-flight response, when triggered by your partner, could be a sign that this person is not good or healthy for you.
If you're ready to end your marriage, you might want to bring your marriage to a close in a peaceful and diplomatic fashion. This will save you a tremendous amount of headache and heartache -- and it will be especially beneficial to your children.