How would losing your memory affect your character, if at all?

I think most of us are familiar with those stories (they usually turn out to be love stories, don't they?) where someone somehow loses their memory, then they basically start life over in a new environment from that point on. For example, if someone lost their memory, woke up in a street alley, and was taken in by a cute little old lady, from that point on they'd probably basically be family with that little old lady and just start living life like normal (with or without the desire to figure out what happened).

Let's just say, for the sake of argument, that they just choose to be like, "Yeah, whatev" and start living life like normal. Would you expect them to develop roughly the same personality, likes and dislikes, and hobbies?

In essence, the question boils down to whether or not these traits (personality, preferences, hobbies, etc) are inherent or learned, or some of both. I'm not entirely sure that the situation I described could actually happen, but I suppose that's not so much the point. :)