Some people aren’t comfortable with the 1.15 line spacing, which Microsoft instituted to improve readability.
To change line spacing, right-click on a paragraph formatted in the Normal style and choose Paragraph. Change the Line spacing field from Multiple to Single. You may want to keep some spacing After in the Line Spacing section to delineate between paragraphs. Now, instead of clicking OK, click Set as Default and confirm that you want this change for all documents created with the Normal.dotx template.