Writing dialogue is a challenge for most writers. There's making it sound “real” or “natural” without it actually sounding the way people talk, which includes constant “uhs,” incomplete thoughts, and sentences that make no sense.
(I discovered that last one in my law practice when I read transcripts of testimony. I was surprised how often people say things that everyone understands in the moment but that is confusing or incomprehensible when read later.)
Then there's the rhythm and musicality of great dialogue. Read or watch a David Mamet play (Glengarry Glen Ross, Oleanna) or Aaron Sorkin show (West Wing, The Newsroom, Sports Night) for great examples of that.
Also, there's character. Just as people don't all talk the same way, your characters should sound different from one another. This difference is not only about how they speak but what they say.
All of which is why this Friday I'm recommending you check out this podcast episode from How Story Works on Character & Dialogue:
Have a great weekend!