In the faith process there are good reasons to believe and bad ones.
Some people come to "faith" out of desperation. Their world caved in on them and they are looking for a quick fix. I'm not saying that a time of crisis is never a time to come to faith. To the contrary, it happens all the time. Often, times of crisis are what opens our eyes and ears to finally hear what God has been saying to us for a long time.
But many people aren't sincere when they come to God in crisis. I see this often as a pastor. People show up to a service or call for prayer or meet me once or twice in counseling. They'll "do anything" for God but what they really want is to rub a genie's lantern and get their wish. When God doesn't perform for them they "lose" their faith. The truth is, they never had faith. They just wanted their problems to go away.
There is danger in desperation not only because it can lead to insincerity but because desperation may cause a person to grab onto a bad faith that promises more than it can deliver. Brian McLaren talks about this type person when he says...
"...they jump on the assembly line of cults and extremist groups, ready to conform, ready to make false confessions, ready to sacrifice their personal responsibilty for the benefit of belonging to a group that is sure about everything." (Finding Faith, page 15)
In times of crisis and chaos, we can be so desperate for an answer that we'll accept any answer as long as it's certain. We may not want to hear, "We don't know how this is going to turn out but you can trust that God will be with you through it." That's often not good enough and so we look for the person who will tell us what we want to hear.
But desperation isn't the only bad reason to believe. Maybe you are lonely and looking for relationships. Maybe you are insecure and looking for affirmation. Maybe you are rebellious and looking to make a statement to your parents or others. Again, faith can answer the loneliness and insecurity issues but if you are grasping at anything to kill the pain then it's not a good reason for faith. Once your need is pacified you'll move on to other things.
Surprisingly, bad reasons to believe don't always end in an aborted faith. Sometimes people are truly converted in spite of themselves! Their mixed motives bring them close to God where God is able to gain their ear and draw them to Himself. But ideally we want to approach God with purer motives if possible.