I've played a lot of games, and when I lose a game of Dota it's absolutely devastating but when I win a good/close game, I am absolutely euphoric, sometimes even into the next day. I don't think (most) people actually take the game very seriously, it just has to do with the mechanics involved. First, it requires a high level of knowledge and focus. There are constant macro and micro strategy decisions to be made and a high level of constant fine motor skills required. Additionally, you are highly dependent on your teammates, as they are on you. If one person on your team makes a big mistake, it can cost your team the game. However, there is frequent opportunity to work with your teammates to set up great plays, which are extremely rewarding to pull off successfully. The third important factor is the match structure. Each game of LoL/Dota is a discrete thing with a small number of participants. It is common in other multiplayer games today to have people coming in and out during a "session". For instance in first person shooters, perhaps you join a 32 player server that is halfway through a map, you play for 15 minutes and then the server changes maps. New people join, some people leave. In LoL/Dota, a game lasts 30-60 minutes and you are stuck with the 5v5 team for that game. I think the extreme highs and lows are at least partially due to the above factors: the level of focus required, the intense team dependence, and the match structure.