If your child is acting like a "sad sack" a little too often these days and you're thinking or saying, "Look, it's not THAT bad!" I have a couple of things for you to try, other than just getting frustrated and trying to help your child feel better with reasoning, again and again.
In the morning write a list of all your child's best personality traits. Think of at least 10 characteristics that you can spotlight throughout the day. Take time to think about this before your child gets up. It's the most important step. Make sure to include a couple about how great your child's voice sounds when they're happy!
Also, practice these words before your child gets up.
"You look like you may feel _______. Is that right? OK, I understand but I don't respond to that tone of voice."
You can be calm because it's such an assertive and powerful statement, there's no need to raise your voice or get emotional. You've acknowledged their feelings and you're willing to help, but you don't respond to that tone of voice or body language they're using.
Now., when they wake up....
Give your child sincere verbal perks based on the list you made earlier. Remember that it's much better to praise a child for effort or for taking on a challenge than for being smart.
"You kept yourself so happy and busy while I made breakfast, Thanks! That was helpful."
"I love the way you sing while you play!"
"Thanks for saying that so nicely to your sister. It sets a great example for her."