What happened was that Gatsby loved Daisy, but because of the war he had to leave and Daisy ended up marrying Tom Buchanan, a very rich man.
After the end of the war Gatsby returns and discovers that Daisy married, and he is determined to enrich (by using illicit means, through the sale of drinks during the Dry Law) to get back his beloved.
Years pass until the two finally meet again, when Gatsby asks Nick to invite Daisy over for tea. As loyal friend Nick does not hesitate and invite her, it is then that the romance comes back to both characters. But Daisy is very hesitant to be able to detach herself from Tom (her macho, racist husband who owns lovers), and Gatsby very determined to have the life so dreamed that for him it sounds like stolen.
And these are the unpleasant things that they have had to face in their relationship.