Did the title get your attention? Good. It was supposed to.
Even free programs tend to come with some "strings" attached.
This one has guidelines and rules, and requirements for quality. If you don't meet those, you won't be approved.
And yes, they do have to approve you.
And no, they don't even have to tell you why they haven't approved you.
AdSense and Google are like any other business, or any other customer you may approach. They can choose to hire you or choose not to. They don't have to provide you with a reason.
Let's say you cut lawns for a living. When you knock on a person's door and offer to cut their lawn, they can say yes or no. If they say no, do you ask them why they said no? Unless you are really aggressive, you probably won't. You'll just move on to the next possible customer.
Why is AdSense any different then? It really isn't.
AdSense does send emails with a variety of reasons why you may not qualify, and it's up to the applicant to figure out why.
If an applicant takes the time to read the whopping amount of information available in the Help Center, they'd probably be able to figure out why.
To get an AdSense account you must meet the quality guidelines, you must have a viable website or blog with lots of content, in some countries you must have six months worth of content, you must not have had a previously disabled AdSense account, you must not have large amounts of copied content, you must not have copyright infringements, you must not have adult, gambling, drug or firearms content or links...and the list goes on.
Every success comes from working hard. People seldom prosper by doing nothing.
If you want an account and haven't been able to get approved, there is help. The Help Center contains all the information you need. The Help Forum has a myriad of volunteers willing to help.
Just don't go into AdSense with an attitude of entitlement. Nobody is entitled to it. Everyone can apply for it.
Whether you succeed or not is up to you.
Read the official adsense blog for ideas on what really works - Inside AdSense