Why does the time traveler make the psychologist press the lever?
he wants his visitors to believe him.
he is too afraid to press it himself.
he can tell that the psychologist wants to do it.

The Time Traveler makes the Psychologist press the lever because, HE WANTS HIS VISITORS TO BELIEVE HIM.

"Look at the table too, and satisfy yourselves there is no trickery. I don't want to waste this model, and then be told I'm a quack"

glamis thou art, and cawdor; and shalt be what thou art promis'd; yet do i fear thy nature; it is too full o' the milk of human kindness to catch the nearest way: thou wouldst be great; art not without ambition; but without the illness should attend it. what thou wouldst highly, that wouldst thou holily; wouldst not play false, and yet wouldst wrongly win: thou'dst have, great glamis, that which cries, "thus thou must do, if thou have it: and that which rather thou dost fear to do than wishest should be undone." hie thee hither, that i may pour my spirits in thine ear; and chastise with the valor of my tongue all that impedes thee from the golden round, which fate and metaphysical aid doth seem to have thee crown'd withal.  

he lacks ambition.  

he is hesitant.  

he lacks courage.  

he has moral scruples.

"the sandy walls seemed to collapse in on themselves"

and

"the bloodshot waters seemed to have no bottom"