Those 'annoying' page breaks

To add a page break into a specific point place the cursor there, then go to the insert tab and then click on Page break.

If you want to remove bage breaks to happen automatically you need to check the paragraph style being applied by going to the style gallery right click on the style being used then click on modify style and then in the format dropdown within the popup go to paragraph. There you can uncheck to add a page break before.

I hope that works!

Word automatically starts a page when it runs out of room. That is, however, not a page break. It is just the text flowing onto a new page. A page break starts a new page regardless of whether the previous page is full.

Stefan suggests using page break before formatting in the heading paragraph style used to begin new chapters. Styles are essential to making Word work for you rather than against you.

References:

• Word Doesn't Know What a Page Is by Daiya Mitchell, MVP
• The Importance of Styles in Microsoft Word
• Why Use Word's Built-In Heading Styles? by Shauna Kelly

If you are using a Heading style for your chapter headings, then you can add "Page break before" formatting to that style. 

Note that there is no way to prevent Word from adding automatic page breaks, based on margins, paper format etc.