This is one of the reasons why I like to study old films. You can see trends that give indications of what was going on throughout the past 100 years or so. One of those is how aliens in scifi are depicted.
During the cold war there were a lot of aliens that were hostile, reflecting the tensions between the superpowers. But some films, particularly those from more liberal filmmakers (what we would call today "progressives") depicted aliens as more friendly or at least not aggressive.
But after the Vietnam War and leading up to Perestroika and Glasnost, aliens in mainstream films became more friendly.
Compare that to Mars Attacks! (1996) which renewed 50's paranoia and applied it to the increase in global terrorism.
Also, since filmmaking is a business, producers don't like to take too much risk, so they tend to imitate styles and trends that have had success at the box office.
