Listening to real-life talk is a first step. From good listening, you become aware of "beats," or the rhythm of dialogue, the pauses, the interruptions, the partial sentences. Learning to listen for beats is essential, but then you have to learn translate the spoken beats to dialogue happens on the page. It has different rules, mostly that the written dialogue lines have to be about 100 times more intense than spoken, because we are not present to pick up all the other cues that reveal meaning.
This is where "subtext" comes in, the undercurrent beneath written dialogue. Subtext is present in spoken speech as well, but much less so. One of my favorite examples is Hemingway's "Hills Like White Elephants," where what's not being said comes forward much more loudly than the words themselves. That's subtext at its best.
But most writers just learning dialogue will dump reveals in every few seconds. They give information and facts in their dialogue, or lots of backstory. Poor choice. Reveal takes away all subtext. And subtext provides both tension and emotion. So you've just made your dialogue the kind that won't get your manuscript a further read.