Why does ophelia kill herself?

It is not by any means clear that she intentionally drowned herself. Gertrude's account of her death suggests that she may have been unaware of her danger. If it wasn't an accident, it seems from the mad scenes that she was disturbed by her father's death "I cannot choose but weep to think they would lay him i' th' cold ground." However, drowning oneself was a traditional mode of suicide for young women with unwanted pregnancies, and this might also be Ophelia's problem, as her mad scenes also show: "Quoth she 'Before you tumbled me, you promised me to wed.' He answers, 'So wold I ha' done, by yonder sun, if thou hadst not come to my bed.'"
It is not by any means clear that she intentionally drowned herself. Gertrude's account of her death suggests that she may have been unaware of her danger. If it wasn't an accident, it seems from the mad scenes that she was disturbed by her father's death "I cannot choose but weep to think they would lay him i' th' cold ground." However, drowning oneself was a traditional mode of suicide for young women with unwanted pregnancies, and this might also be Ophelia's problem, as her mad scenes also show: "Quoth she 'Before you tumbled me, you promised me to wed.' He answers, 'So wold I ha' done, by yonder sun, if thou hadst not come to my bed.'"