1. More money.
2. More prestige or responsibility.
3. Lack of promotion at current employer.
4. Lack of respect.
5. Burnout.
6. Boredom
7. Disinterest.
8. Issue with colleague(s), boss, or manager.
9. Recently completed school in another field.
10. Recruited by a new company.
There are times when everything is getting you down. Work isn't going well. Perhaps you are burning the candle at all ends. You are mentally, physically, and emotionally exhausted. You are at the end of your rope. Simply put, the straw dropped that's breaking your back. Here are a  few suggestions on handling burnout:
-Try to sleep at night. You are more productive when you are well rested.
-Avoid drugs and alcohol.
-If able to, take a vacation or a PTO day to relax and recharge.
-Speak with a professional to work on stress reduction.
-Call a friend or family member to vent.
-Strive to balance your work and home life.
-Exercise. (I hear it releases endorphins. I'm still trying to find the time!)
Also, speak with management to work cohesively to prevent burnout before it gets the best of you.
Just because you want a job or you think you can do a job does not mean that you are the right fit for a company or the specific position for which they are hiring. If a job requires a year of experience and you are a new graduate, the manager is not waiting for you to graduate, get licensed/certified, to hire and train you. There is a reason they are looking for an experienced person. Perhaps the position will be a mentor for others. Maybe they don't have time/resources to train. To apply will be both disappointing to you and a waste of time to the employer, who is looking for a targeted type of person.

If you are getting a bunch of fast rejection letters or emails, take a look at the jobs to which you are applying. Objectively think about it from the hiring side. If you were paying for someone to take the job, would you hire your background on paper? If not, apply to more jobs within your scope. Get experience through volunteer work.

Understand that the company is not waiting for you. They are looking for the person who will best fill the role for that job!