Changing My Goal?

I am finding it difficult to not change my goal weight.  My initial goal weight was 140, but then I changed it to 130.  But now as I get closer to my goal weight I want to change it again.  It's like I am trying to set myself up to never reach a goal and I'm not sure why I keep doing this.  For now I am leaving my goal at 130, but there is a good chance I might change it to 120 as it would be a very difficult goal to reach.  But I like being challenged and this might be a good challenge for me.  I know that I don't want to go under 120 because for my height I will probably look too skinny and I don't want that.