In the beginning of a relationship—the honeymoon phase, the newness creates an increase in good feelings, excitement, passion, and interest in one's partner. After a while, when this phase settles down into the "real" part of the relationship, this heightened emotional state naturally decreases. As it decreases, so does the motivation to impress one's partner.
Generally, this can lead to feeling less passionate within the relationship. Often when one loses interest in the passion and/or sex, there is also a lose of interest in caring about looking good or sexy. This can cause one to grow too comfortable, stop taking care of themselves, and then gain unnecessary weight.
The good news is this does not have to happen!
When we are conscious of this dynamic we can avoid it altogether or undo it if it has occurred.
The first step is to create a good relationship with ourselves. This entails looking at, addressing, and healing all the parts of ourselves that are operating from a dysfunctional place. Many people accomplish this by reading self-help books or seeking professional help from an expert.
The second step involves taking personal responsibility for our choices. This means that we do not blame another or a circumstance for where we are but accept our part that got us there. This is very empowering as it helps us to make good decisions and take positive productive steps to keeping ourselves healthy.
Finally, and maybe the most important is application and follow through. One can acquire all the information and help possible, yet without actually putting it into action nothing will change. Trying to change everything all at once never works because it becomes too overwhelming.
I encourage my clients to do this with a "baby-step" mentality meaning one little step at a time. Start with one small thing to change each day and follow through by applying it. For example, if you want to work on taking better care of your body, you make one-step towards that at a time. Perhaps if you drink five sodas a day, you can lower that down to four until you are comfortable with three, two, one, etc.
To get you started I highly recommend these self-help books!