In this sentence the wordshadowsrefers to a reflection. Some men are so much in love with themselves that they kiss their own reflections (like Narcissus in the Greek myth). Egotistical self-love, the words suggest, will not lead to bliss or happiness.
The use of silver for this particular casket is explained with the following lines:
There be fools alive, iwis,
Silvered o’er; and so was this. (67-68)
Just because a man gets older does not necessarily mean that the man gets wiser. Even men with gray, white, or silver hair can be fools. The wordthisalso refers to the casket. The Prince of Aragon, thus feeling very much a fool, then quickly leaves Belmont.
At the end of the scene, a messenger arrives to inform Portia about the coming of Bassanio. The third attempt to win Portia’s hand will soon be made. The third time will be the charm.