Please explain use of subjunctive when the subject does not change in the main clause to the subordinate clause.

Hola Ralph

With verb creer in negative we can use both the infinitive or the subjunctive (even if it is the same subject). You could say:

"Yo no creo ser muy egoista" (I don't think I am very selfish)

or

"Yo no creo que sea muy egoista" (I don't think I am very selfish)

(The second sentence is more commonly used)

The rule about using the infinitive instead of the subjunctive, when having the same subject  doesn't apply to all verbs. 

With "wish" verbs like querer, this rule if fixed:

Yo quiero ir a Madrid (same subject, infinitive)

Yo quiero que tú vayas a Madrid (different subject, subjunctive)

But it is not the same case with "creer". 

I hope this helps,

Inma