I suppose this would make more sense with a little context, for instance, the previous sentence. But even without it, for might mean, roughly, "because" -- as in
/.../ Aye, there's the rub, For in that sleep of death, what dreams may come /.../
So I'd suppose that before your sentence, the talk was bout someone reacting in an unusual way after having been rejected, or feeling like killing themselves. Or it may have been about the need to show some patience or consideration with a person who has been rejected. So it could be paraphrased as ...
Because there is no doubt that being ignored or rejected by someone you’re enamored of is associated with all kinds of distress.