Should a human be happy all the time? 
 Answer 1: Happiness is not only subjective but also temporal - what brought you joy 2 years ago might be boring/unpleasant now. So, instead of optimizing for happiness at a given time, it might actually be better to realize what makes us feel down or not content and avoid them. By avoiding such circumstances/conditions, we can be content and maybe that's what need in life: not strive to be happy all the time but avoid what we truly don't like so that happiness takes care of its own.

Answer 2: Yes! Life is too short to get caught in dull moments so a human needs to optimize for maximum happiness. The definition of happiness differs and that's ok; we all live once and we should spend our time looking for new things that make us happier.

Answer 3: No. Most meaningful lessons in life are learned in situations when we're not particularly happy: Loss of a close person, financial break downs, unexpected health conditions. Getting through these hard times is actually makes a person stronger and more prepared for other calamities in life. We should not strive to be happy "all the time" -  instead we should take things as they come. In life, there is a need for moments of discontent as well as a need for moments of joy.