Dating someone who is not the one for you

If you are dating someone and you are content in the relationship. For the most part you are happy and you do enjoy being around them, however you know that this is not something that you want long term and that you enjoy their company for the time being however your different mindsets and ways of dealing with stress and other main factors let you know with out a doubt that this relatuonship is just "for now" do you tell that person that!? Not every relationship is headed towards marriage and it seems like something that might be taken as an insult but i also worry about being honest about where it is i stand so that not to have them invest more of themselves then they probobly should