How do you know that Mary is happiest in her husband's presence?

Mary Maloney, one of the most powerful characters of Roald Dahl, is the protagonist in the story "Lamb to the Slaughter." From the beginning of the story Mary is shown as a typical housewife who patiently waits for her husband by the door. Even after being six months pregnant, she gets up to greet her husband Patrick Maloney, and serves after him. She takes his coat and offers him a drink and after realizing that he is tired and troubled she is ready to cook in the house even when its the day they usually dine out. "It is a blissful time of the day" for Mary to sit in his presence after the longs hours alone at home, she loves "to luxuriate in the presence of this man; and to feel almost as a sunbather feels the sun, that warm male glow that came out of him to her when they were alone together" Few more details in the story reveal Mary's happiness with her husband and her satisfaction of their marriage.
"She loves the way he sits in a chair, the way he comes in a door, moves across a room; the intent far looks in his eyes, the shape of his mouth, and his silence."
These few of the many instances reveal Mary's happy and content life and because of reasons like these the sudden decision of Patrick, to abandon Mary comes as a shock to Mary and as well as the readers.

Readers know that Mary Maloney is very happy in her husband's presence because the story fairly overtly tells readers this information. When Mary is introduced to readers, she is patiently awaiting her husband's return from work. She greets him at the door, takes his coat from him, and returns with a drink in hand, serving his every need. In the following paragraph, readers are then told how much Mary loves the following moments when she gets to just sit down and "luxuriate" in Patrick's presence.

For her, this was always a blissful time of day. She knew he didn’t want to speak much until the first drink was finished, and she, on her side, was content to sit quietly, enjoying his company after the long hours alone in the house. She loved to luxuriate in the presence of this man, and to feel—almost as a sunbather feels the sun—that warm male glow that came out of him to her when they were alone together.

We don't know why Patrick gives her the news that he does, but it is possible that he feels smothered and overly confined in Mary's presence; however, it is still shocking to readers that he could so coldly end a partnership with someone that so clearly adores him. It's also quite amazing that Mary is able to move so quickly from practically worshiping the ground Patrick walks on to killing him, establishing an alibi, and disposing of the murder weapon.