Before the baby turns 8 months of age or older the baby doesn't recognize the kisses as affection as much.
After they turn 8 months of age or older then the baby begins to recognize the kisses and love as affection.
Babies also don't like being held by strangers as much either before 8 months of age and they will cry and let you know they don't like that stranger even if they're relatives or the babies grandmother or grandpa.
When the baby gets to 8 months or older then they'll usually understand why you're kissing them or hugging them and know that the kisses and hugs mean love to them and they'll show love back to you.