The Open Source Ideology

In the development of software, open source signifies that a product includes permission to use its source code, design documents, or contents.
There has been conflicting definitions and differences between the free software movement and the open source movement.

The term “open source” software is used by some people to mean more or less the same category as free software. It is not exactly the same class of software: they accept some licenses that we consider too restrictive, and there are free software licenses they have not accepted. However, the differences in extension of the category are small: nearly all free software is open source, and nearly all open source software is free. 
-- Free Software foundation  http://www.gnu.org/philosophy/categories.html

Though the market for open source software is on the high, there is still a school of thought against it.
There are many reasons projects should go open source, a few are:

By uploading your codes to sites like GitHub, it makes your code available to people with similar ideas to yours. People can help improve your project and probably make suggestions to to guide you.

Writing a complete software is an arduous task. Lets face it, our ideas can't always be the best. By allowing others access to our software, we can get have the knowledge and expertise of our contributors. Going open source leads your software to be of a better quality.

Going open source doesn't make your project free, you can keep some parts of your code as private as you want it. People have the notion that making their codes available would give their competitors the upper hand. But consider that you have a lot of contributors to your project which will far outweigh what your competitor can get.

I hope these few tips has changed your perception about going open source. You can go around and explore some open source software or start making your project open.