Summary
Using extra letters while texting is a way to emphasize words (similar to the exclamation point). It is also a way to seem more friendly whilst texting.
Texting Uses
Extra letters are almost exclusively used by teenage girls, but can also be used (generally in moderation) by anyone who is texting.
Context
If someone is using many extra letters while texting you and is using a happy tone, this generally means that they are happy to talk to you and trying to seem like they are more friendly or more comfortable with you.
Examples
Seeming more friendly: “Heyyyy”
Emphasizing emotions: “Whyyyyyyy” “Wooooooo hoooooo!”
Textetta Stone 