It seems to like getting its own output fed back:

YOU> HI! HOW ARE YOU DOING?

C64> BELON

YOU> WHAT DO YOU MEAN BY THAT?

C64> YOU DES

YOU> YOU TOO!

C64> YOU DESER! THIS THE SHIN

YOU> DESER? SHIN? THOSE AREN'T EVEN REAL WORDS, WHAT'S GOING ON HERE?

C64>

YOU> THIS THE SHIN

C64> YOU MAKE ME HAPPY TOO

YOU> YOU DESER

C64> YOU MADE MY HEART! W

YOU> DES, DESER, DESERST?

C64> YOU DES

YOU> BELON

C64> I LIKE OURON ME! WONDERFUL!YOU!

YOU> YOU BELON DES

C64> YOU MAKE ME HAPPY