They have a new style of dressing and buy more expensive clothes. Some even start going to the gym because they decided that they want to look better. Your partner starts looking at his/her reflection in the mirror more often or starts visiting cosmetic studios. For whom do they want to look better? Normally if you ask them about this, they will answer that they want to look better for themselves, but that is not what is happening.
This is just a cover-up, they want to look better for someone else. There once was a documentary about how a husband started cheating on his wife by visiting the local brothel. He took her money and instead of spending it to pay the bills he spent it in the brothel. His wife noticed that over the period of a few months he started to keep track of his diet and lost some pounds. It certainly is a great motivation to lose weight because of a lover.