Imo, lentils need carrots the way food in general needs salt, so that might have been the difference. I assume other sweetish (culinary) vegetables like squash or sweet potatoes would work as well, but since I tried them with carrots, I haven’t made them without to test (seriously, it’s that big a difference, plus carrots are the cheapest of the three).

It doesn’t need to be much, just one or two roughly chopped and thrown in with the raw lentils to cook together. I don't generally want the mushy carrot mixed in with the lentils afterward, so I just eat it separately (which is why I chop it roughly- if you chop it finely, it’d probably basically melt into the finished dish).