I know Budapest! In the comics, Black Widow used to offer certain ~services~ to enemies to get them to reveal information, then kill them so they couldn't say anything (like the actual spider). She met Hawkeye in Budapest, but decided not to kill him after getting the info because she liked him. Then there was all the fights and everything else. So Black Widow remembers Budapest for the fights and stuff, and Hawkeye remembers Budapest for the... ( ͡° ͜ʖ ͡°)
-Twisted_Soul