This is a topic one probably spends little time thinking about. Galaxies are gigantic objects that are just exist, right? Well just as people have a certain intrinsic core to their personality which is complemented and modified by their interactions with other people, from a certain point of view the same can be said of galaxies.
Galaxies have intrinsic qualities too such as a huge number of stars, likely as many or more planets, gas and dust (and an still larger amount of dark matter). Galaxy shapes and even their properties and the future of their ability to form new stars is strongly influenced by other galaxies near to them. The ‘neighbors’ tug on one another gravitationally, which has the effect of stripping off the outer layers of gas. This, in turn, typically acts to takes away the interesting spiral shape we normally associate with galaxies (or at least with the most beautiful examples of galaxies).
The Milky Way retains its superb spiral shape, despite being very old and therefore likely to have suffered a great many interactions with neighbors and even direct collisions in its long lifetime of about 13.5 billion years.
In fact, a galaxy called the Large Magellanic Cloud may be falling into the Milky Way today (well, it is a long process)! By now the Milky Way is so massive that the only event that can possibly change its shape significantly is a direct collision with another galaxy of roughly equal mass.
The Milky Way will collide head-on with its nearest massive neighbor called the Andromeda galaxy in about 5 billion years. When the collision happens (long after we are gone), one will not need a slow motion setting on your camera as the collision will take not one second as it takes to watch two football players tackle each other. Instead, it will take a few hundred million years to watch this collision happen from start to finish.
The result will be a giant football-shaped galaxy called a ‘giant elliptical’ galaxy. By this point the personality of the Milky Way will be changed it that it will probably stop forming new stars every year as it does today. Let’s enjoy this 5 billion years while we can!