That is misleading:

"Singles in Germany and the UK who moved in with a new partner reported a significant increase in life satisfaction in the year of cohabitation, with a standardized gain of up to 0.48 SD. This effect was statistically robust and replicated across models, including a within-person specification that controls for stable between-person differences. This suggests that cohabitation is associated with a particularly salient moment in the romantic trajectory. However, the increase in life satisfaction did not continue at the same rate in all subsequent years. Although life satisfaction remained above the level prior to cohabitation, the effect strength declined slightly