Act 1, Sc. 4, lines 66-80: Why doesn't Horatio want Hamlet to pursue the Ghost?

Horatio: Do not, my lord.

Hamlet: Why, what should be the fear? I do not set my life at pin's fee. And for my soul, what can it do to that, Being a thing immortal as itself? It waves me forth again. I'll follow it.

Horatio: What if it tempt you toward the flood, my lord? Or to the dreadful summit of the cliff That beetles o'er his base into the sea, And there assume some other horrible form Which might deprive your soveriegnty of reason And draw you into madness? Think of it. The very place puts toys of desperation, Without more motive, into every brain That looks so many fathoms to the sea And hears it roar beneath.