To each their own. In simple lists for navigation menus I always omit closing li. There is no ambiguity on what I am intending even with those closing li omitted in such simple cases:

  <nav id=main-nav>
  <ul>
  <li><a href="/">Home</a>
  <li><a href="/hamburgers/">Hamburgers</a>
  <li><a href="/sausages/">Sausages</a>
  </ul>
  </nav>

<p> indicates a new paragraph. <li> indicates a new list item. Unless otherwise specified, the existing paragraph/list item continues. There's nothing magic about any of this, it's part of the HTML spec.

Laziness doesn't play a role. This isn't XML where you need to repeat yourself over and over again or abusing a bug in the rendering logic; it's following the definitions markdown language you're writing content in.

If you're not too familiar with the HTML language then it's always a safe bet to close your tags, of course.

The personal judgement isn't really helpful here?

If you don't close your <p> and <li> tags, you risk accidentally having content in the wrong place.

It's something to avoid because it can have bad consequences, not because it (somehow?) makes you a bad person.