Even the most successful relationships can go through periods of predictability and reduced intimacy. A stagnant phase can be one of the last stages a relationship goes through before it comes to an end, or it can just be a temporary plateau in a relationship that keeps growing and becoming deeper.
All stagnant relationships were once dynamic, exciting and new. Otherwise those two people would never have become intimate enough for the relationship to get stuck in the first place. Any long-term relationship has to balance feelings of excitement and enchantment with stability and comfort. However, too much comfort can equate to predictability and boredom. If you feel stuck in a pattern of reduced intimacy and growing distance, you need to change how you interact with each other.
To revive the feeling of novelty in your relationship might be as simple as suggesting a fun new activity the two of you can do together, such as going on a road trip or taking a dance class. Surprise your partner by meeting him at work or leaving a clever note in his wallet for him to find. Make reservations at a favorite restaurant or plan a date night. Having more fun together can completely change the quality of your relationship. After all, you decided to become a couple in the first place because you enjoyed each others' company. If your partner isn't open to your first idea, ask for suggestions.
When a relationship becomes stagnant or predictable, one of the first things to go is usually physical closeness such as cuddling. It's easy to get so caught up in errands, projects and other distractions that you stop spending time together just holding each other and talking. If this aspect is missing from your life or not at the level it once was, you may have to work at getting it back. Start by scheduling time to be close to each other and sticking to the schedule. Once you're back in the habit, it should become spontaneous again.
Petty negativity is one of the biggest obstacles to getting a relationship back on track. Both partners get stuck in a pattern of sarcasm, criticism and complaint instead of warmth and affection. Start giving your partner more positive feedback in every way you can think of -- support instead of sarcasm, praise instead of criticism, expressions of gratitude instead of complaints. This doesn't mean you should ignore or repress feelings of unhappiness in the relationship. However, if you can increase the fun and physical closeness in your relationship while reducing the negativity, both of you will probably start feeling a lot better. This should make it much easier to have a productive conversation about where the relationship is going and what you both want.