I must admit, personally, I am not quite sure whether there is a certain universal formula for this but I guess there are things that definitely help in keeping babies happy and hopefully grow up to be positive and happy people. Undoubtedly, various sources would have some slightly different opinions or suggestions but here is what comes to my mind as a mother of two and as a person who reads and discusses about kids related topics fairly often:
Physical Needs: Even the richest or the most famous person in the world can't enjoy their luxurious lifestyles or the items that normally make them happy unless their tummies are full and the need to go to the toilet has been met. First thing first! Make sure your baby is not hungry, overtired, sleepless, cold, hot, wet, has a soiled nappy on etc. (I think most babies are quite happy once this stage is successfully passed which is not always the easiest thing in the world, I must admit-especially sleeping). So, be responsive to your babies' needs and read the signs well.
Emotional Needs: How many of us don't need or want to feel some warm hugs, hear some softly spoken nice words, look into the eyes of a person with love that can be read in their eyes too and so on. Babies are the same. They need to feel that they are loved and cared for and are secure. I am not sure who can't get enough of hugs and feels happier in our case; me or my babies/children? So, give your baby hugs, touch/rock them gently, be connected with them.
Right Atmosphere and Environment: Of course some calm music etc would be helpful but I am actually referring more to a safe, fight free (as much as possible) environment with a good vibe in general here. Consistency is a significant need too. Last but not least, make sure you provide her with lots of opportunities to have fun.
Other: Playing together is a great way to keep babies happy now and in the future I believe. I don't mean play together at all times but I do mean often. They sure need some time alone or with others and it is a very important part of their physical, social, emotional and mental development. However, playing with you and feeling the responsiveness and affection help them build confidence which also leads your baby to be an independent, resilient and a happy person. Furthermore, it is also significant that you talk to your child. Regardless of the age of your precious one, your child will benefit heaps from your talk and the soothing voice of yours. It is never too early to start talking to your baby! Yes, you can and should definitely start talking to your little angel even before the birth, when pregnant.
Please feel free to add or discard any suggestions in the comments section if you believe that is needed.
Enjoy your lovey-dovey day with your baby today!