I`ve been told this before and it does not offend me unless it is being said accusingly, as if I am making things difficult for them. Someone being hard to approach has nothing personal to do with with individuals wanting to approach, its an impersonal decision and shouldn`t be taken as a personal slight.
Only some people think I am hard to approach, most people think I am aloof but if they approach me they find I am very approachable and willing to converse. I just don`t seek out random social connections to establish security, I`m comfortable in my own world. If someone is finding it difficult to approach someone else it isn`t entirely about the approache, something in the approacher creates that difficulty and the person they are attempting to approach is not responsible for that.