Leonard and Penny’s relationship has been a major subject in the series since the beginning of season one pilot episode. Most of the season finales involve Leonard’s relationship with Penny in some way.
When Penny first met Leonard, she only thought as him as a neighbor and a friend and had no romantic feelings for him at first. She asked favors of him and talked and visited him nearly every day. She ate take-out food with him and the others. He took Penny out for dinner in “The Fuzzy Boots Corollary”; however, Penny thought the whole gang was coming. When no one else showed up she suspected their evening was supposed to be a date. At the end of the season 1 after another bad break-up, she confessed she wanted to go out with a guy like him and with that, Leonard asked her out which she accepted. Their date went well though Penny wanted to take it slow. Afterwards, Penny gets the idea that he won’t date a girl that doesn’t have a college degree derailing a second date at that time.