In Hamlet, when the company arrives to watch the play, Hamlet hints that the cause of his madness is both ambition and unrequited love. How and why...

Hamlet suggests that his condition is caused by various factors in his 'To be or not to be' soliloquy in scene 1, Act III. In this dramatic monologue, he rhetorically asks who, in his right mind, would be able to bear all the burdens and heartache that life imposes upon him. He compares such a situation to being in purgatory - caught between heaven and hell. He specifically mentions that one of the most painful conditions that one has to bear is to suffer the pain of unrequited love. He states, in part:

...For who would bear the whips and scorns of time, 
Th' oppressor's wrong, the proud man's contumely, 
The pangs of despis'd love, the law's delay, 
The insolence of office, and the spurns 
That patient merit of th' unworthy takes, 
When he himself might his quietus make 
With a bare bodkin?...

The burden, to him, should be too great to bear and one should, therefore, choose suicide as a release from such pain and anguish. He suggests, however, that such a one (implying himself) would only hold back from committing such a deed if he is fearful of the consequences of suicide, as can be seen from the following:

Who would these fardels bear, 
To grunt and sweat under a weary life, 
But that the dread of something after death- 
The undiscover'd country, from whose bourn 
No traveller returns- puzzles the will, 
And makes us rather bear those ills we have 
Than fly to others that we know not of?

It is clear that Hamlet has greater fear for the unknown and would rather face those unbearable conditions which he has to deal with since they give him certainty. He would rather deal with what he knows than have to contend with what he does not.

Later, in his conversation with Ophelia, he directly mentions that he is ambitious:

I am very proud, revengeful, ambitious; with more offences at my beck than I have thoughts to put them in, imagination to give 
them shape, or time to act them in.

He is, in effect, telling Ophelia that he he bears more ill will than even he can imagine. His malice is so great that he would run out of time before he had even committed all the evil with which he is filled. His damning declaration obviously shocks Ophelia who later declares:

O, what a noble mind is here o'erthrown! 
The courtier's, scholar's, soldier's, eye, tongue, sword, 
Th' expectancy and rose of the fair state, 
The glass of fashion and the mould of form, 
Th' observ'd of all observers- quite, quite down! 

She pities her wretched state for she had previously enjoyed 'the honey of his music vows' and has now just witnessed Hamlet's fall from grace. He has 'fallen out of tune' and is 'harsh.'

Hamlet is obviously aware that he is being monitored. He is being deliberately harsh and outspoken because he wants others to believe that he is irrational. He knows that Ophelia will report his behaviour to her father, Polonius, who would then report to Claudius. He is also more than likely aware that Polonius might be listening in on his conversation with Ophelia.

His behaviour is a deliberate ploy, a distraction, just before the play is presented. Who would believe that someone in his condition would have the clarity of mind to fashion and deliberately present a performance that implicates anyone, least of all the king?

It seems as if Hamlet's ruse worked to a certain extent, for Claudius, who had been eavesdropping with Polonius, later comments:

Love? his affections do not that way tend; 
Nor what he spake, though it lack'd form a little, 
Was not like madness...

and, at the end of the scene:

Madness in great ones must not unwatch'd go.

Polonius contends:

...But yet do I believe 
The origin and commencement of his grief 
Sprung from neglected love...

Claudius, however, has seen Hamlet's condition as the ideal excuse to get rid of the young prince, for he tells Polonius:

...I have in quick determination 
Thus set it down: he shall with speed to England 
For the demand of our neglected tribute.

The plan would be to have Hamlet executed on his arrival in England, but that is another matter.