Self-publishing can sometimes work. If you’re only interested in selling to family and friends, or if you have some sort of sales platform which will help promote the book, then you might do OK.
Self-publishing allows you to retain control over every aspect of your book: but you can’t be an expert in every area, and so your book is not going to be as high-quality as those edited and designed by experts. The physical quality of many POD books is low, too, which means that if you want a good-quality book you’ll have to consider offset printing, which comes with a high associated cost.
You’re probably not an expert sales-person, either, and so you’re going to struggle to sell it. You’ll be lucky to get it into your local bookshops, but have no chance of getting it stocked nationally (and despite Amazon’s global coverage, most books are still sold out of bookshops). Most self-published books sell fewer than 100 copies, while for a commercially-published book sales of 2,000 would be pretty poor. And while you’re working on your sales, how can you devote your time to writing the next one?
Finally, remember that once you’ve published your book you’ve used up the first rights to that book. Publishers are only usually interested in acquiring first rights. They’ll not want your self-published book unless you can prove quite extraordinary sales—last time I heard, that meant sales in excess of 5,000 in just a few months.
Despite all this doom and gloom, don’t let it put you off if you’re absolutely sure it’s the way you want to go. Just don’t imagine it’s the gateway to wealth, fame or writing success.