Multiplayer games provide a completely different environment for the player to experience the game and his or her personal gameplay. By having other people in the game alongside the player, it allows him or her to experience the game differently than he or she would if playing solo. While there could be greater feelings of accomplishment when completing something with a team, the effects of failure could also become much different.
When playing with others, it can become quite easy to revert to assuming yourself to be the "perfect player" who makes no mistakes and only benefits the team's goal. While this may be the case sometimes, it always has to be the fault of at least one person for the failure of the team. By human nature, one never believes that person to be him or herself unless he or she is honest and sees his or her mistakes and accepts them. However, many people are not this honest with themselves. By seemingly "having done no wrong" in the failure of the team, it becomes easy for the player to become frustrated or upset with his or her teammates because they were unable to succeed within the game. Although the intensity is much lower than, say, a first-person shooter, Stardew Valley could have this effect if players within the game are not completing tasks at a desired rate or not performing the most efficiently.
Not only does this effect present itself during co-op play, it can be even more prevalent in versus play, namely team versus play. For example, Rocket League, a game I am much more familiar with, offers team play against another team of the same number. You then work as a team to play "car soccer" against the other team and try to score the most goals. This game induces many levels of frustration within oneself and with the opponent's success, but these frustrations can quickly turn to your teammate(s). Due to the nature of humans, frustrations drift away from yourself (because nobody wants to be the result of their team's failure) and onto your teammates. This creates tensions between the player and his or her teammates, leading to a much less enjoyable playing experience.
Mario Kart can provide a similar experience for the player. In general, racing occurs as a solo competition against all other players. Although driving ability comes into play in Mario Kart, having power-ups creates an excuse for the player to blame his or her opponent for his or her lack of success in the race. Driving ability quickly fades from the mind of the player, and he or she focuses on the other drivers assaulting him or her from all directions.
In conclusion, multiplayer games show that players tend to become more distracted on the other elements of the game that cause them frustration rather than the actual gameplay, which in turn leads to anger that may have been avoided had the same game been played alone.