Here's the deal. Nagging does not work. It just doesn't. And no matter how gentle your comments, your loved one will probably see it as nagging. The major thing you have to accept here is that your loved one is not you, and you can't make him or her do anything they truly don't want to. I watched my parents struggle with their smoking habits for years. My mother would quit, Dad wouldn't, and soon Mom was back to smoking again. Dad wasn't going to do it until he really wanted to; until he had a reason that spoke to him and gave him the will to follow through and keep following through. My mother got breast cancer and finally, they both quit, together, and have been smoke free for a couple of years now (Mom is also currently cancer-free). No matter what any of us said or did before that, Dad didn't really want to quit, and so he didn't. He would try half-heartedly now and again, but he didn't really want it and so he always fell back into it.
There are some things you can do to help a loved one reach that motivation point when they have the will, desire and follow-through to change their health habits, but first accept that it may not happen right away, or for a long time, or possibly ever. Remember why you love the person and focus on that. You might even tell them some version of "It drives me absolutely crazy that you don't seem to care about this and it makes me worry about you. But I love you no matter what." Oddly enough, sometimes this reassurance is all someone needs.
Some things you can try:
-ask the person to exercise with you. Make sure you're doing something that's reasonable for them, I.e. if you're a marathoner and your spouse is a couch potato, try walking with them, not suicide sprints.
-if they have expressed interest in exercise but haven't followed through, ask if they would like to work with a professional to figure out what to do and get in the habit. Then encourage them to work with a trainer, or purchase a package with a trainer as a gift. (Don't do this is the person is resistant; it will just be a waste of money if they are not ready to commit.)
-try to help the person find something they will enjoy. For example, there are several video games for fitness out there, like the Wii-Fit. If your loved one hates organized exercise but likes video games, maybe this will help. If they hate treadmills but love basketball, encourage them to join a team, join one with them, or start playing with them.
-if your loved one feels too frazzled and you can help, do so. Example - if they feel too busy with the kids, volunteer to watch them while they head for the gym.
-if you're looking for help and support in your own health journey, let your loved one know that. Sometimes we will go further to help our loved ones out than to help ourselves.
Remember, you want your loved one to exercise because you care about them. Be patient and understanding, as you would want them to be about your own faults and bad habits. Change happens gradually, not overnight.