Why do I need a lawyer to get married ??

You don't "NEED" a lawyer, however, it is highly recommended. Immigration is a complex area of law and his highly fact dependent. There is a great deal at stake. A good attorney should be able to assist you in putting together a thorough package that will facilitate speedy processing, and set your case up well for the additional steps that may come down the road - depending on the circumstances of your case. e.g. removal of conditional status, or obtaining citizenship. I've been retained many times by people who started the process themselves and then created problems since they were unaware of issues.

In short, you should be concerned about what it is you do not know about immigration law. There may not be any issue, but each case is highly fact dependent. Hope this helps clarify the concerns.