A warm welcome, Annakova! Help yourself to the refreshments.
There's a rant around here somewhere about that, but I'll just reiterate. One of the most important parts of writing is to make sure that what you're saying can be easily digested by the reader, which is why saying things plainly ("white eyes stared at him through the dark") is more suggested than using prose ("two misty, glowing white lights gazed at him through the pitch black of the room"). You don't want the reader to have to stop every few words to look up something because that takes them out of the immersion of the story.
The nice thing about the word "said" is that your mind automatically skips over it in favor of who's saying it, thus keeping the flow intact. You should never underestimate "said" - it's your best friend in writing. You can use certain words in place where they fit (mumbled, screamed, whined, etc), but overall, "said" is automatically ingested and the story can continue.
So when a story is constantly using other words in place of it, that halts the reader quite often, as well as becomes obnoxious (we get it, Hunter, you're writing cats. How about you actually write cats instead of reminding us six times a page?). It's as if the writers were afraid of repeating themselves in the one place that you can and should, but ended up repeating themselves anyway.
Let's take your example:
"Hello," he lilted.
"What's up?" she growled.
"I don't know," he theorized.
"Well, let's go do something!" she threatened.
You have to stop and recognize what emotion and tone each of the descriptors are trying to convey, but if what's being said isn't actually conveying much of an important tone, then it's just a waste of everyone's time.
There's a lot that goes into writing dialog, but the one rule I should make clear is to not be afraid of "said".