Often, they end happily with the main couple getting together. For example, in 'Fruits Basket', Tohru Honda and Kyo Sohma end up in a loving relationship after all the trials and tribulations they've been through.
However, there are also sad endings. In '5 Centimeters per Second', the main character Takaki Tono has a series of unfulfilled loves. The story ends with a sense of melancholy and longing as he and his childhood love, Akari Shinohara, have drifted apart over time.
Some monster anime stories end with a revelation about the origin of the monsters. This could lead to a new understanding between the humans and the monsters, resulting in coexistence. For instance, if the monsters were created due to some scientific experiment gone wrong, and in the end, the humans find a way to reverse the process or live in harmony with the monsters.
Often, one person in the love triangle ends up with the main character they are meant to be with. For example, in 'Inuyasha', Inuyasha finally chooses Kagome over Kikyo. Another way is that the character in the middle realizes they don't have romantic feelings for either of the other two in the triangle and decides to be alone or just be friends, like in some less - common storylines.
Often, they end happily with the couple getting together. For example, in 'Fruits Basket', Tohru ends up with Kyo, and they have a future together filled with love and understanding.
They often end happily. For example, the couple gets together and lives happily ever after, like in 'Fruits Basket' where the main characters find love and also resolve their family issues.
Often, they end with the main couple getting together. For example, in 'Kaichou wa Maid - sama!', Misaki finally realizes her true feelings for Usui and they end up as a couple.
In some cases, they simply fizzle out. The initial excitement of the physical relationship wears off, and they both come to the realization that they are better off as just friends. There's no big drama or falling in love, just a mutual understanding that the 'friends with benefits' arrangement isn't working for them anymore and they go back to being regular friends.
Some end happily, with the two young people becoming a couple and having a sweet relationship in middle school. For instance, they might hold hands and go to the school dance together.
Some end happily, with the lovers finding a way to be together across time. For example, in 'Outlander', Claire and Jamie manage to have a life together despite the challenges of different time periods. Others might end bittersweetly. In 'The Time Traveler's Wife', Henry's death is a sad but also an inevitable part of their story, yet their love remains strong. And then there are those that end with the characters making sacrifices to preserve the integrity of time, like in some more complex time travel romance stories where changing the past too much could have disastrous consequences.
Some end happily. For example, the teacher and student might find a way to be together after overcoming all the difficulties, like in the story where they both move to a new place where their relationship is accepted.