When two people interact for a prolonged period of time, the less dominant one tends to copy (mimic) behaviors from the more dominant person. Such behavior is not only good as a way to achieve the feeling of trust and understanding between the two people but it is also an excellent way to learn new skills or try new behaviors.
This characteristic can also go into the negative specter of things. For example, one might pick up the habit of smoking, drinking, gambling or just plain emotional abusive characteristics. This means that you should not be afraid if you find that you have remained the same as from the start of the relationship, but if you find yourself going to dark places that you couldn't even imagine before the start of the relationship, then by all means either talk with your partner or just get out of that abusive relationship. This is very important according to relationship experts.