There's a difference between doing things for God and loving God. Here's a question I received from one ministry leader I work with. It's a great question that I wish more people were concerned with.

"Every year I  have at least one girl in my group that does QT [daily devotions] 7 days a week.  I challenge them to do more by reading and studying other verses on the same subject.  They never do.  TIME is the issue and when they have time they want down time (their time).  I don't want QT [daily devotions] to be routine or something just expected and since they are good kids they do it. I'd rather it be something they can learn from.
 
The same with teens that [memorize scripture and are actively serving].  They do things to the expected limit then use school as a reason to not read more books because they have enough reading in school."

Here is my response to her:

"This is a great question. Many leaders don’t think about the fact that just doing Quiet Time or memorizing verses doesn’t mean that someone is close to God. You definitely have the right goal in mind for your group.

Practically speaking, it might be good to challenge your students with a specific goal that is a small step. For example, if they say they don’t have time to read more, try challenging them to spend 5 extra minutes on top of what they would normally do. That’s enough time to read a Proverb every day or a chapter of a book in the new testament. Then after a while you can challenge them to increase their time by 5 more minutes with a different reading idea (like a Psalm per day). Sometimes it’s easier to do things when we have a really clear plan laid out for us.

The other thing that’s really important (as you pointed out) is that they see their Quiet Times as more than just something they do. It is time for them to spend with God. Someone who is passionate about God will want to spend time with Him. That doesn’t mean that they will spend 3 hours with Him every day necessarily, but it does mean that time with God is something that is looked forward to.

For example, when my wife and I were engaged, we didn’t spend all of our time together. But we always wanted to be together: we looked forward to it and when we were together we wished we had more time. Our love for God should be no different. No one is perfect, so we won’t always be as passionate about God as we should be, but it needs to be our goal.

There’s a few different ways you can help your girls to have that kind of passion. First is to make sure that you are modeling that passion for them. You will never convince your students to be passionate about God if they don't see the same passion in your life first. Second is to purposefully teach them to think about their relationship with God as they would a relationship with a guy that they were in love with. They should have the same passion for God as they would for their boy-friend or fiancé, or husband eventually.

Lastly, you may need to approach some of them one-on-one and ask them about their relationship with God directly. If you feel that they are lacking passion for God, you can lovingly confront them about it. Start by asking some probing questions to determine where they are at (we have to be careful not to assume we know how things are). Then be honest with them about your observations. Lovingly tell them that you feel they are lacking passion as motivation for their Quiet Time and other spiritual disciplines. Then, depending on how they react, you can help them change that."