Act 1 scene 3 ends with Iago's soliloquy. Why does he think that this plan may be succesfsful in Othello?

Iago's plan in Act 1, Scene 3 of Othello is brilliant.  He
understands people very well, and he knows what do to do to instigate conflict.  He is
quite suspicious of Othello, who he believes is sleeping with Emilia.  He is angry with
Cassio for taking the position of lieutenant that Iago felt belonged to him.  So, Iago
needs to take revenge on both Othello and Cassio.

Perfect
revenge has several characteristics:  the victim must suffer more than the avenger did,
the avenger must exact his revenge with impunity,  the victim must know that the avenger
is causing his suffering, and lastly, the avenger must be present to see his victim
suffer. Iago's plan for revenge would satisfy these characteristics nicely with one
exception:  Iago does not plan to make his victims know that he is the avenger.  Iago
prefers to catch his victims as a spider traps his prey; they will fall into an
invisible web and the more they struggle to extricate themselves, the more ensnared they
will become.

Iago's plan is to make it seem as if Desdemona
and Cassio are having an affair.  Cassio is a "proper," or handsome, man, according to
Iago.  He is "framed to make women false."  A man such as Cassio could easily arouse
jealousy in other men.  Cassio is in fact young, eloquent in speech, and white--all
qualities Othello lacks.  Further, Iago knows that Othello trusts him, that Othello is
of a "free and open nature" and deems others honest, because he himself is.  Iago is
confident that Othello will believe him when he suggests that Desdemona is sleeping with
Cassio.  After all, Othello has known Iago much longer than he has known Desdemona.  Why
wouldn't Othello believe him?

In this way, Iago can ruin
Cassio, for surely Othello will fire him if he thinks he is sleeping with Desdemona, and
he can destroy Othello's marriage as he thinks Othello has destroyed his.  The plan is
brilliant in its simplicity.