Perhaps your girlfriend frequently complains about you being a sourpuss when she's around. Or maybe you regularly feel stressed when you're around her. In either case, it's time to adopt several strategies to boost your happiness. Not only should you aim to stay happy to keep your relationship lighthearted and fun, a more positive outlook on life can also be good for your health.

Schedule Worry Time

You might find yourself unhappy around your girlfriend because your mind is worrying about other matters. Persistent worry not only hinders your social interactions, it can also damage your mental health. Learn to limit your worries to 15 minutes a day, suggests psychology professor Daniel Freeman in the Psychology Today article, "Don't Worry, Be Happy." If worries pop up outside of that designated "worry time," such as when you're with your girlfriend, push them off and save them for later.

Express Your Gratitude

Remembering to express gratitude can boost your happiness, suggests psychologist William F. Doverspike in the Georgia Psychological Association article, "Gratitude: A Key to Happiness." Before you hang out with your girlfriend, list several things about her you are grateful for. Perhaps she is a great listener, or she keeps you laughing. Now try to picture life without her and everything you would miss out on. When you finally see her, you should feel more enthusiastic and attentive.

Be Active Together

Physically active people tend to experience more pleasant feelings than people who aren't, reports Amanda Hyde, a kinesiology graduate student and researcher, in Amanda L. Chan's Huffington Post article, "Exercise Makes Us Happy -- It's Science." Hyde's study, which tracked the daily activities and moods of 190 students, indicates that you and your partner can boost your happiness by staying active together. This is because physical activity triggers the body to release endorphins, which are chemicals that cause you to feel an enhanced sense of well-being. Go for bike rides or daily jogs together. Not only will you reap psychological benefits, but also physical benefits.

Laugh Often

Laughter reduces stress, boosts your mood and even strengthens relationships, reports HelpGuide in the article, "Laughter is the Best Medicine." This is because, like exercise, laugher also releases endorphins, leading to a natural boost in mood. Laughter also reduces stress hormones, relaxes your muscles and increases your body's immune cells, which helps you fight off infection. When you're with your girlfriend, always look for opportunities to add humor into your interactions. Develop inside jokes, watch funny shows or share hilarious stories about your day. Do some light teasing, but don't go too far, otherwise you risk souring the mood.