It's likely the way you are approaching it.
I'm guessing you are asking her questions, and she doesn't feel comfortable with them.
If you hear something you want to explore, try offering her a similar experience that you had, and how it made you feel. Then let her talk.
I think though, that perhaps you are overthinking things. If the deep topics are sad/bad, then there are many reasons for people not to discuss topics that are too deep with people they really like:
- they don't want to discuss sad topics with you
- they want the friendship/relationship to be about positives
- they want a person they can just be happy around