The love interests in Chuck are never introduced merely for angst and in order to delay the will-they-or-won’t-they dynamic. They are introduced to advance Chuck and Sarah’s relationship and to have them learn something from the experience.
Lou showed up in mid season 1, when Chuck wanted a real relationship, pushed Sarah to become jealous, and advanced Chuck and Sarah’s relationship by forcing her to admit, even to herself, that she had feelings for Chuck.
Then both Chuck and Sarah needed to get over their past relationships with Jill and Bryce respectively.
Cole shows up at the perfect time, at a moment when Chuck wants to move the relationship forward and Sarah is stalling. Cole, with his glamour, bravery, and sexiness, symbolizes the best of the spy life and represents The Last Temptation of Sarah. When she rejects him, she subconsciously rejects the spy life in favor of a normal life with Chuck. Chuck witnesses her temptation and realizes things are difficult for her as well, so he stops whining and decides to get the Intersect out of his head in order to be with her. This leads to the awesome Orion arc that follows.
Hannah and Shaw were introduced to show Chuck and Sarah that they couldn’t go back to their old ways—a normal relationship for Chuck and a spy relationship for Sarah. They had changed each other too much to go back to that.
