Iago is in military service under Othello. He is, technically, his "ancient," which is a corruption of the word, "ensign." In Elizabethan English, this meant "standard-bearer"--an important distinction since Iago has to carry Othello's identifying flag with his shield and colors and represent him.
Why is this important? The military hierarchy and where Iago fits into it seems to have something to do with Iago's hatred of Othello, who is not his "master," but rather his commanding officer and the one he publically represents. He does say:
I follow him to serve my turn upon him.
We cannot all be masters, nor all masters
Cannot be truly follow'd.
But continues with:
In following him, I follow but myself.
So, though it bears some similarities to a master/servant relationship, the "following" that Iago is doing is in a military system in which both Othello and, more recently, Cassio are above him in rank.
As for reasons, his reasons are evasive and not direct, like the quote given above. He follows Othello, but follows himself. Not a very clear statement. At the end of the scene he says:
Though I do hate him, as I do hell's pains,
Yet, for necessity of present life,
I must show out a flag and sign of love,
Which is indeed but a sign.
So, it could be deduced that he follows him to keep his job and keep food on the table ("for necessity of present life").
Yet, it may not be useful to look too closely at what Iago says about why he is doing anything. He warns Roderigo (and the audience) about his nature in line 64. He says, "I am not what I am." Though Iago seems to give reason that it is necessary, at present, for him to follow Othello, it is also hard to believe anything he says. What Iago's reasons are for anything he does is a question that has been debated over and over again through the centuries.