Sedevacantism does not come from being sad, lonely or isolated. Rather, it comes from devout Catholics who possess both common sense and logic.
If a Catholic believes in all of the papal powers and then observes what has been going on in the Church for almost 70 years, then common sense and logic tells said Catholic that the papal seat must be vacant because the men claiming to be popes for almost 70 years have absolutely contradicted what was taught, believed and practiced since the beginning of the Church almost 2000 years ago.
Sedevacantism is the current state of the Catholic Church and any denial of it (no matter how uncomfortable it may feel realizing this unfortunate truth) simply leads to a destructive "cognitive dissonance" (i.e. a contradictory state of mind) in the mind of a Catholic who does so.
This is as simple as I can explain it.