How can I be happy at other's happiness? How can I be happy at other's happiness ? How can I develop my willingness to see others' being happy ?

If I define a happiness level, scale of 0 to 100 about being happy at others' happiness, I think I'm at the 25-50 level. How can I promote it to 90-100 level ?

Sometimes, I do loving-kindness meditation. But I'm looking for some kind of understanding / wisdom about being happy ? How can this help in path to the enlightenment ?

Or Should I do loving-kindness meditation to gain that wisdom ?
If you have a child then when your child is happy you are automatically happy, and that happiness you experience is 100% without holding back, that is because your love and compassion for your child is unconditional.  The same type of happy feeling you must get if your thoughts are pure towards others happiness.  Jealousy is the reason why you cannot feel 100% happiness for others when they are happy. You need to develop loving kindness and compassion towards others what we call Metta meditation. 

Every day 30 minutes in the morning and 30 minutes in the evening you can practice Metta meditation. Find a suitable place - calm environment in your house. Sit and close your eyes and stay focus by saying a prayer, if you are a Christian say a prayer few times and bring your attention without allowing your mind to wonder, then when your mind is calm you say, "May I be well, May I be happy, may I be joy, may I be safe, may I be peaceful", repeat this three times, then you say the same but to your immediate love ones, "May my closest family and friends be well, may my closest family and friends be happy...", etc., three times, ( you must concentrate and have feelings when you are saying these words),  after that you say three times, "May my normal family and friends be well, may my normal family and friends be happy...", etc., last you say three times, "May my enemies be well, may my enemies be happy...", etc., finally you say, "May all beings in this cosmos be well, may all beings in this cosmos be happy, ...", etc, three times, then you start all over again from the beginning. When you start doing this for few months, you will gradually develop your loving kindness and compassion, you may see changes especially if you have a pet at home it helps, because every living being you must have compassion and loving kindness, you will soon start to have compassion towards even a cockroach and will not want to harm, you let it live and have compassion for it's misfortune of being born as a cockroach.

You must also learn to let go of things that you normally attach to, and it is easy to start with material things, eg, you might like to wear only expensive brands of clothes, you will need to change the mindset of not making yourself important by wearing ordinary not so expensive clothes, this is a way to change your mindset which helps you to let go of things.

You can gradually let go of other things little by little, until you feel there is no competition between you and others, until this happens your jealousy will not disappear, that means your happiness level to go up you need to gradually get rid of your jealousy ( which arises due to ego). You can gradually increase the times of your Metta meditation.

Wisdom what is discussed in Buddha Dharma is not the same kind of wisdom what the west talks about.  

May you be happy and well