People who have high debt are generally ashamed that they're in this position in the first place. They feel really guilty about having to carry it into their new relationship. If the person with the debt can acknowledge faults and take responsibility for past actions, he/she will work hard to pay off all amounts that are due. On the other hand, if the person cannot readily accept his/her role in the situation and refuses to take any ownership of it, it may be time for you to re-evaluate the relationship.
Over all, when you're planning to spend the rest of your life together with that special someone, you'll need to determine whether you can accept his/her past debt or whether it's better for you to move on from the situation. A person with a high amount of debt could have hundreds of thousands of dollars to pay back to a creditor or lending agency, and it could take several years just to pay down the interest. It could affect you as a couple when buying a house, a car or when your spouse applies for a job. Even the simple luxuries that couples enjoy, such as going out to a movie would have to be negotiated as the person may not be able to afford to go. Just remember that you are accepting your partner (and his/her debt) as a complete package deal.
If you do decide to accept this person into your life for the long haul, try not to judge your partner on his/her past spending habits and be supportive instead. This certainly isn't an easy task, but having you as a trusted partner will make the journey easier. Helping someone overcome a past debt can lead to tremendous growth, healing and understanding.