In Shakespeare's Hamlet, Act One, scene two, lines 70-75, Gertrude speaks to her son, asking him to behave nicely about her remarriage to Claudius:
Good Hamlet, cast thy nighted color off,
And let thine eye look like a friend on Denmark.
Do not for ever with thy vailed lids
Seek for thy noble father in the dust.
Thou know'st 'tis common. All that lives must die,
Passing through nature to eternity. (70-75)
Gertrude and Claudius are speaking to Hamlet who is suffering deeply from the death of his father, Old Hamlet. Gertrude asks Hamlet first to shake off his gloomy looks. In line 72, "Denmark" refers to Claudius, the new King of Denmark. Gertrude implores her son to be friendly toward her new husband, and we can infer that in his mourning (and he does mention starting on line 79) he does not think highly of his stepfather.
In line 73, Gertrude advises her son not to search the ground (having his sad eyes lowered) for his father. In lines 74-75, his mother offers flimsy reasoning that death is not unusual—every living thing must die at some point. It is the way of things pass from this life into eternity upon one's death.
These are Gertrude's arguments as to why Hamlet should not be sad, or unfriendly toward Claudius.
It is important to note that Hamlet is not just grieving, but he is extremely displeased for two other reasons. First, his mother's hasty marriage took place scandalously soon after Old Hamlet's funeral. Second, during the Elizabethan era when this play was written, people believed that when married, a husband and wife become one—to the point that there was no separation between the two in spirit or in body. They believed that the essence of the one came to reside in the other. With this understanding, Hamlet (and the audience) would have been disgusted, for marrying one's in-law was committing incest. In other words, when Gertrude sleeps with Claudius, that part of Old Hamlet that still resides in her is actually also sleeping with Claudius...incest. This was a big, big deal at the time and would have created a great deal of friction between the characters of Hamlet and Gertrude, and between Hamlet and Claudius.