Just as a chain is only as strong as its weakest link, a relationship is only as happy as its least happy member.
It is worthwhile to pay attention to your spouse’s happiness level (in life in general, not only about happiness in the marriage). Of course, you can’t make someone happy; their happiness depends on the level of joy they allow in themselves. You can, however, change what you do in your life and for/with them; this can impact how they foster their own happiness.
Reflect about what might be causing their lower level of happiness. Might it be overwhelm? Is it something that you are doing or not doing for them? Might they be feeling a lack of personal fulfillment? Do they have time and money to spend on things that are meaningful to them? Do they have local friends and the time to spend with them?
Think about ways that you can help them in those areas that might be impeding their happiness and their overall wellbeing. If you notice that you are speaking to them in ways that cause distress, change your words and tone. If there is something that you can take off their plate, offer to do that. Offer support as they work through a next step in life. Spend more time together (or in some cases, less time). Bring more playfulness into your lives.
While you can’t make them happy, you can create an environment that can foster their happiness in life.
When your spouse is happier, then the whole relationship is happier. Do your part in bringing happiness to your home.