By default, when you are entering content into Sitefinity and you hit the 'enter' key, Sitefinity will add br tags. This is less than ideal. What you really want is p tags.
For example, by default, each time you hit 'enter' a br tag is added, and your content looks like so:
Here's how it looks with the default turned off, and a P tag used for line breaks.
It's also worth noting that if you are using P tags but you want a BR tag within a P tag, just hold the Shift when you hit Enter.
Sitefinity is super configurable, and they have an option for this, you just have to go change it.
If you were curious, using P tags is also the default in the HubSpot blogging tool.