My husband is young and is in a nursing home. Why is it he crys when ever he sees me?

Jeannegibbs gave you a wonderful answer. This is so sad for you both.
Likely his frustration and feeling of loss are overwhelming. None of this is about you or your daughter. It's very hard for teenagers to understand when a parent or grandparent doesn't know them. Let's face it - it's hard for adults to deal with this. It can feel like rejection. Please keep talking with the daughter and maybe see if the school counselor can offer her some support or suggest a group for teens who are going through this. Try to remember that he can't help any of this and get support from others who are going through the same thing. You may want to try the Well Spouse Association at www.wellspouse.org. Take care,
Carol

It doesn't matter what age a person is when dementia strikes, it robs them of many basic cognitive abilities we take for granted. Many dementia patients lose the ability to recognize loved ones and other familiar faces. Please help your daughter understand that this is not personal about her. Her dad cannot help this flaw in his mental abilities at this point. Even if he doesn't know who she is, he can know that she is a caring and loving young woman who visits him.

It is hard to know "why" dementia patients do specific things. Maybe he cries because he does still recognize you and knows how much he has lost. All you can do is show him your love and that you are there for him.

My heart goes out to you, dealing with this tragic disease at such an early age.