Quote 1
OPHELIA
My lord, as I was sewing in my closet,
Lord Hamlet, with his doublet all unbraced;
No hat upon his head, his stockings fouled,
Ungartered, and down-gyvèd to his ankle,
Pale as his shirt, his knees knocking each other,
And with a look so piteous in purport
As if he had been loosèd out of hell
To speak of horrors—he comes before me.
[…]
He took me by the wrist and held me hard.
Then goes he to the length of all his arm,
And, with his other hand thus o'er his brow,
He falls to such perusal of my face
As he would draw it. Long stayed he so.
At last, a little shaking of mine arm,
And thrice his head thus waving up and down,
He raised a sigh so piteous and profound
As it did seem to shatter all his bulk
And end his being. That done, he lets me go,
And, with his head over his shoulder turned,
He seem'd to find his way without his eyes,
For out o' doors he went without their helps
And to the last bended their light on me. (2.1.87-94, 899-112)
Elizabethans thought that love really could make a man sick and mentally ill. They called this state "love melancholy." Check out what a doctor, Bernard Gordon, had to say in Lilium Medicinae:
The illness called heroes is melancholy anguish caused by love for a woman. The cause of this affliction lies in the corruption of the faculty to evaluate… [men forget] all sense of proportion and common sense…it can be defined as melancholy anguish. (source)
Here, Ophelia describes Hamlet as looking and acting just like a guy who's playing the stereotypical role of an unrequited lover. It's a textbook case of lovesickness. In fact, maybe too textbook—almost as if he's read the book, if you know what we mean.
Quote 2
OPHELIA
I shall the effect of this good lesson keep
As watchman to my heart. But, good my brother,
Do not, as some ungracious pastors do,
Show me the steep and thorny way to heaven,
Whiles, like a puffed and reckless libertine,
Himself the primrose path of dalliance treads
And recks not his own rede.
(1.3.49-55)
After Laertes warns his little sister to keep her legs closed, Ophelia points out the double standard at work in Laertes's advise. In other words, our girl's not afraid to tell her bro that he's got no room to talk about chastity, especially given that he's been running around like a "puff'd and reckless libertine." Ophelia's remarks here also demonstrate that she's not necessarily the wimp some literary critics paint her to be. Here, she gives as good as she gets. So, why does she end up drowning in a brook?
Quote 3
OPHELIA
By Gis and by Saint Charity,
Alack and fie for shame,
Young men will do 't, if they come to 't;
By Cock, they are to blame.
Quoth she, 'Before you tumbled me,
You promised me to wed.'
So would I 'a done, by yonder sun,
An thou hadst not come to my bed.
(4.5.63-71)
We'll let literary critic Carol Thomas Neely handle this one: when Ophelia goes mad, her disturbed language sounds a lot like patriarchal oppression (the oppression of women by men) (source). Take this son: it's about the loss of a maiden's virginity (she's "tumbled") and a broken promise of marriage. Just like girls in almost any historical era, she's stuck: if she doesn't have sex with the guy, he'll dump her for being a prude; if she does, he'll dump her for being—well, not a prude.