If happiness and suffering existed externally, no matter how long you stayed in the sun it would just get more and more pleasant.  It’s not like that.  The present pleasant feeling itself becomes unpleasant.  The objects of pleasure and displeasure do not exist from their own side.  That is why the first lines of the Four Seals of Buddha’s Teaching are so important:
All compounded things are impermanent.
All contaminated things are suffering.
All phenomena are empty and selfless.
Nirvana is peace.
That second line, “All contaminated things are suffering,” has vast meaning.  Buddha did not say happiness of the senses is bad, don’t enjoy it.  It’s just that happiness which arises from external objects of the five senses is not real happiness.  Don’t be attached to it.  Don’t be fooled by it.  If we chase after it with attachment and craving, like an elephant stuck in the mud or fish caught on a hook, that temporary happiness takes away our freedom.   It does not mean those external objects are bad.
As the Mahasiddha Tilopa instructed Naropa:
Son, appearances don’t blind you, craving them does!
Cut off craving, Naropa!

Written by Khenpo Karten Rinpoche in the book Precious Teachings, the chapter entitled “Happiness and Suffering Are Only Appearances, They Are Not Real” page 102 in the paper version and 108 of the PDF version.