Reasons first (more or less).  This summary is fairly close to the order of presentation found in the text selection:

No one, including me, wants to do what would likely cause harm to himself.  If I corrupt my associates (that is, make them wicked), they are more likely to harm me.  So, clearly, I would not deliberately corrupt the youth of Athens, who are my associates.  Therefore, either I do not corrupt them, or (if I do corrupt them) I do so unintentionally.  If I do not corrupt them, then I have not violated the law at all and should not be on trial; and if I have corrupted them unintentionally, then I should not be on trial; for the law aims at punishing deliberate, not unintentional, wrongdoing.  Unintentional wrongdoing calls for assistance and correction, not punishment.  Either way, there is no basis for me to be on trial.  If you, Meletus, had thought carefully about these matters, then you would see that you shouldn’t be putting me on trial as you are.  Thus you haven’t given much thought to these matters.
 

Conclusions first (more or less).  This summary does not follow the order of the text very closely, but it still presents the basic reasoning quite well:

Meletus has not thought much about charging me, because if he had, he would see that I shouldn’t be on trial.  That’s because the law is for punishment of deliberate wrongdoing, and I have either not corrupted the youth at all, or else if I have it was not done deliberately.  For no one wants to do things that will harm himself, and if I had deliberately made the youth wicked (i.e. corrupted them), then I would have likely caused harm to myself.  So, either I haven’t corrupted them at all, or any corruption I caused was unintentional.  If I corrupted them unintentionally, then Meletus should help me correct my ways, not punish me for deliberate wrongdoing!