going with other people

 There's nothing wrong with going on vacation alone, I did it a couple of times and it was, well, lonely.

One very good reason to have a partner is that if you disappear alone nobody will come looking for you right away. Also it's twice as hard to make two people disappear so your chances are twice as good that you won't disappear, maybe.

Also any number of problems can come up that make it very convenient to have extra people involved. You may become seriously ill, or your financial preparations may stop working for some reason. For instance if you encounter fraud someone may try to run off with all your money.

There was one person who lived alone but found a vacation partner for going on sightseeing tours. They had similar interests and went places to see historic sites or just fun places to visit.

If you are specifically going for some type of event like surfing or skydiving or something where you're going with a random group of other people then maybe going alone works better. Or if you're the person that likes hiking and exploring alone and having oneness with nature or something, then it probably makes sense. You may have noticed in the news every once in awhile though that sometimes when you have these oneness experiences with nature, nature kills you.