In Reports › Lead Closed Reasons, you can see a break down of the reasons you’re closing out a lead or job. For example, it may be that you just never heard back from them and the lead went stale, or maybe it's because you were over their budget.  You can customize your closed reasons in Settings at Settings › Job Closed Reasons.

You can find the Closed Reasons Report in the Reports sidebar menu of your account.  Note that in order to view the Reports section, you must be an administrator user.