When he says a "heart--how shall I say?--too soon made glad" he means that she was too friendly with others, especially men. The narrator did not like the fact that she was "too easily impressed" by others or that she liked everyone and everything. Although she was most likely just a happy and sweet woman all around, he didn't like her giving anyone else attention--especially men. She seemed to be so thankful for what others did for her that she "thanked" them all. And he alludes to the fact that she may have been unfaithful when he says,
"she thanked men--good!but thanked somehow--I know not how"
What she should be thankful for is the aristocratic name he gave her when he married her. She should never have been so thankful to others as she was just to be his wife.