I have a friend who is very content. She doesn't strive and she is very comfortable. BUT, she and her husband earn enough for her to be a part time worker only (she could stop work all together, but she likes to 'dabble' (what I wouldn't give to 'dabble') and likes her field of work, so chooses to keep working part time). Her and her husband have a combined huge income and can afford holidays, two cars, a camper van they use on weekend trips away, all the extra-curricular stuff their chiildren need, treats out, AND, they want for nothing. They are not millionnareis, but they are very very 'comfortable' (don't have to worry about money).
If I was in my friends situation I would be happy as I am too, and not wanting to strive for more. I would be pleased with my lot and not wanting more.
However, reality is that DS is a talented swimmer and I need to pay for his lessons and club which are v expensive. My children need school uniform and I need to pay for that. We need to pay our mortgage, and I want to take my children on holiday and enable them to do fun things like go camping, go to Legoland etc....
So no, I am not happy as I am, and yes I do feel I always need more (mainly money, but an extra car would be handy, and a bigger house so DS2 has a room as big as his brother)
When I win the lottery I'll be comfortable and will stop striving for more material things. Until then I can't help wanting more for my children (maybe if I didn't have kids I'd be fine not to strive too)