When you copy and paste text using Apple-C / Apple-V you are not only copying the textual data but also the styling of the text. This is not a good thing most of the time as it would create a mix of different font types, colors, weights, etc. in your document and you have to manually reset to make it look consistent.
You can however use a simple trick to bypass the pasting of stylistic info and just paste the textual data. Just use the Apple-Option-Shift-V shortcut to paste and avoid extra work.