Jesus asked to be baptised so that he could make a public act of repentance for his sins.
What?! Are you saying Jesus wasn't without sin?!
Yes. The Word became flesh and it is absolutely impossible to be flesh without sinning. It's just the way the world is. Even if he managed to avoid all deliberate sins he would have been guilty of innumerable accidental (in the true meaning of the word) sins and a near infinite number of sins of omission.
You want Biblical proof?
Well, there was the incident in the Temple when he was twelve years old. Don't make excuses for the lad! What he put his parents through that week was "unforgivable."
You want conclusive proof?
When Jesus said "He who is without sin, cast the first stone," he didn't throw a rock at the woman, did he?!