Be honest. How supportive are you towards helping your partner reach their fitness goals? In my experience as a personal trainer, it is clear that family and friends have a huge influence on an individual's probability of success in achieving their target, from weight-loss to running a marathon.
Let's face it - any goal worth achieving involves an element of sacrifice. If someone is devoting a lot of time to training, this could mean spending less time with their partner. But it doesn't always have to! It is fantastic when I see other halves adopting the "if you can't beat 'em, join 'em" philosophy when their partner starts a new fitness training regime. They find that the time spent together - cycling, running or walking, for example, can turn out to be the best quality time they have spent together in a long while: enjoying the great outdoors together free from the incessant interruptions of modern technology. It can work well even if you are at different fitness or motivation levels - a partner accompanying the occasional long run on bike, for example, may be an easier sell than them undertaking all your marathon training!
Support towards a fitness goal doesn't stop with the training - a partner's influence can be even more important when it comes to food shopping and mealtimes. If you are lucky enough to have a partner who does most of the cooking, don't expect them to cook two separate meals if you don't fancy the healthy food that they are preparing (yes, believe it or not this does happen!). Also, if you know your other half is trying to lose weight, don't tempt them with offers to order take-away!
If you are really honest, does part of you worry that you may look bad in comparison if your partner succeeds? Why not see their ambition as an opportunity rather than a threat? Get on board and exercise more and eat more healthily. You will always be much stronger as a couple working together towards a common goal.