Love is not synonym of happiness. Greatest misconception about love is that it is happiness in disguise. Even sex is not good everytime, let alone love.
Love is blend of all possible human emotions. Love,like, hatred, jealousy, misunderstanding,pain,passion, compassion, benevolence,lust, company, insecurity, security and all kinds of contradicting emotions. You are some times happy as well as angry from same person at once!!
This is why love is a rollercoaster ride of contradicting emotions. This is why love is thrilling. It full fills our all needs of emotional variety. Love is complete emotion like a rainbow.
Hence happiness is a small part of love kind of subset.