Why is that when I down vote a post then 2 of my reputation gets deducted?
You lose 1 point for downvoting an answer and none for downvoting a question. This is because downvoting hurts the post's owner. We want you to only downvote when you seriously believe it deserves it. We don't want people to downvote simply because they feel like it or don't like the author or don't like that they used "your" instead of "you're" somewhere in a 1000 word post.
That said, it is only a loss of 1 rep point for downvoting an answer because we realize that reputation is hard-earned and we don't want to discourage free expression of opinions; 1 rep point is not much at all.
Downvoting is not to be taken lightly, so this is status-by-design because we want you to take it seriously but not be discouraged from using it. -1 for downvoting a post gets the point across but doesn't hurt too much.
That's the way the system works. You should have a look at the available help pages, e.g. this one:
You lose 2 points when someone downvotes one of your questions or answers and you lose 1 point when you downvote someone else's answer.
A downvote symbolizes your disagreement with someone's work. It is literally saying, This answer is not useful (hover your mouse over the downvote, that is the alt-text that pops up). The loss of 2 rep for you suggests that the decision to downvote should not be taken lightly (i.e., just downvoting for the heck of it) that you should truly judge whether the post really is wrong or misguided.
As an aside, you gain 10 rep when your answer is upvoted and 5 when your question is upvoted. Unless you are downvoting >5 answers for each answer-upvote (or >2 answers for each question-upvote) you receive, you are still going to gain reputation based on simple math.