This is not to say that I believe anyone is ever 100% happy. Over pizza last night, I realise how true that really is. People will always want more. When we already have so much, our yetzer hara kicks in - whether it's wanting a boyfriend/girlfriend, wanting to be married or wanting children, wanting a bigger house/nicer clothes/that new pair of shoes youve spied, another holiday.
And when we have nothing, we want for basic things that many simply expect from life - fresh water, a roof over your head, food, a life without disease, without fear.
But you can be content. Happiness, according to the theory that underscored my year in sem, is finding your purpose in life. When you discover what it is Hashem intended you to do, you can be happy - you have a goal, your life can be purposeful, you have a reason to exist. Some, like Yonah, try to run from that purpose. But eventually it'll catch up with you. When you realise your potential and your purpose, and you live your life with that purpose as the framework, happiness can be achieved.
So rarely can this be found. But everyone can feel a degree of contentment, in some way, at some pt in their lives.

shabbat shalom!