If you read Facebook's advertising policies, you'll notice how strict they are.
There are hundreds of reasons why your ads may be disapproved. But according to Facebook, the top three reasons for disapproval are as follows:
1. Too much text in the image
If your ad contains more than 20% of the image area covered by the text, your ad cannot be broadcast. It is always better to reduce the amount of text in the images of your ads, if possible.
Facebook has a handy tool called the Text Overlay Tool, which allows you to upload your image and see if it fits. I suggest you do this if you are unsure about the specific images you want to use.
2. Age limit
The second most common reason for disapproval is that the advertisement is shown to an audience that is too young. For example, alcohol ads must meet certain age guidelines based on local laws.
This means you need to target people over 18 in the UK and over 21 in the US.
These restrictions are part of Facebook's "restricted content" policy, which includes things like dating, gambling and supplements. You are allowed to advertise these things, but you must do so in a way that respects Facebook's policies.
3. Mention of the "Facebook" brand
The third most common reason for disapproval of Facebook ads is the mention of "Facebook" in your ads. You are allowed to use "Facebook", but you must follow certain rules.
Unless you're advertising a product related to using Facebook, you shouldn't be interested in the latter. You can learn more about the ad approval process on the Facebook Business site.