How does Hardy demonstrate constantly in his works that man is destined to suffer?

Hardy, though he lived a very normal, contented life, promotes the idea of fatalism or determinism in all his works.  While there is almost always a "higher power" in his works (referred to as Providence in a true Victorian sense), that higher power does not intervene in humans' lives; thus, they are forced to wander around aimlessly, suffer meaninglessly, and usually die amidst cruel circumstances.  No matter what Hardy's heroes and heroines do; they are fated to suffer.  Here are several examples:

1. Far from the Madding Crowd: Gabriel Oak is a decent man who "happens" to spot Bathsheba, a woman around whom the rest of the novel revolves and who becomes inextricably linked with Oak despite her own wishes.  No matter how much Oak tries to help Bathsheba, Fanny, or others, circumstances just do not turn out right.  Bathsheba loses her husband after finding out that he has fathered a child with Fanny, and the wealthy man who longs to marry her loses his mind.  She ends up marrying Oak who asked her at the beginning of the novel despite her statement that she would not do so.  Fanny, is trapped in her poor circumstances, and it seems that no matter how much she chases happiness, the weather and society clash with her desires, and she ends up dying pennyless and loveless.

2. The Return of the Native--Beautiful Eustacia desires more than anything else to escape the heath which entraps her. She lies, cheats, and does whatever she thinks is necessary to marry Clym so that he will take her away, but he ends up marrying her and staying at the heath.  Even when she tries to run away with Wildeve, she ends up dying along with the innkeeper.  Clym, who tries to do good for his community, loses his mother, wife, and sight, and lives out his life in the same small area where he grew up.

3. Tess of the D'Urbervilles--Tess is the most literal example of Hardy's belief that man's destiny includes suffering.  Unlike the women in Hardy's other novels, Tess does nothing to bring about her suffering.  She follows her parents wishes, works hard, tries to please her "relative" Alex, all to no avail.  She loses the one man she loves, and just as they meet again, she has killed her husband and is soon after executed.