Ecclesiastes 1-3 Meaning Of Things And Time

How can the king, son of David say that “everything is meaningless”?  Why is this in the Bible?  What is being compared?  Since “the eye never has enough seeing nor the ear its fill of hearing” (1:8) how can there be “nothing new under the sun”? (1:9)  What is the alternative to “under the sun”? (“over the sun”?)  How might 2:24-26 really be “meaningless” to some people?  In ch. 3 – what is “the burden God has laid on man”?  Does this chapter teach “eat, drink and be merry”?