What makes an RTS so appealing is the sense of control it provides. Unlike games in other genres, you arent in control of just a single individual, or in some cases, a small party of individuals. Instead, you are put in charge of an entire army. From the lowly footsoldier, to rumbling tanks, to the giant battlecruisers, they are all yours to command. You have all the say in what every unit under your control does, and its up to you to decide how you are going to accomplish the task at hand. In an RTS, there is no set formula for victory (aside from kill all your opponents stuff) but you are the one that determines how the other guys stuff is going to get destroyed. Due to a commanders playstyle and favored units, how they play the game can greatly differ from someone else.
Its the control over how you are able to play the game that keeps you coming back for more. Whats that? Youre getting bored with a certain strategy? Feel free to try a new one which can freshen up the game for you. Have a problem with Zerg Guardians destroying your base defenses? There are several strategies you can use to get around that. RTS games are a tacticians playground, and you can experiment with differing strategies to your hearts content. Its all about how you play the game, not how the game plays you.