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I found this on Wikipedia and before anyone starts screaming that it's not a relevant source-which it isn't always-this one just addresses WHY Cleris fans believe in Cleris while why Cloti fans believe in Cloti.

Final Fantasy VII/Romantic Arguments/Serious version
From Final Fantasy Wiki

The "love triangle" between Cloud, Aeris, and Tifa is probably the second largest debate about Final Fantasy VII, and, like the Aeris resurrection theory, is still argued to this very day. Throughout disk one the player must make decisions about who he likes more - Aeris or Tifa. This results in the "date" scene at the Gold Saucer; however, you may also "date" Barret or Yuffie by making Cloud friendlier to one or the other.

Despite the fact that it is up to the player to decide who he loves many people debate over it. They have names for each couple. Cloud & Aeris are called either Cloris/Clorith or Cleris/Clerith while Cloud & Tifa are called Cloti. Many fanlisting websites for those two couples have sprung up; however, a lot of "flaming" websites have sprung up as well. Square Enix has been very keen not to be direct about who the canon couple is because it would alienate a lot of fans.


Reasons People Believe In Cleris/Clerith

1. Cloud was clearly upset by Aeris' death.
2. He killed Sephiroth to avenge Aeris.
3. Aeris was the first person Cloud started being warm to.
4. In all of Yoshitaka Amano's artwork, Cloud is only paired up with Aeris.
5. In "Advent Children", he wants forgiveness from Aeris for being too slow to prevent her death.
6. Aeris is the easiest date to get. You must alienate Aeris to get Tifa.
7. Cait Sith foretells that Aeris and Cloud are a perfect match.
8. Though Aeris sees a resemblance between Cloud and Zack, she says, "But you're different."
9. During "Advent Children's" end credits, a love song plays. Cloud and Aeris are the only characters shown.
10. At the end of the game, Cloud is shown extending his hand to Aeris.
11. Cloud becomes Aeris' bodyguard in exchange for one date.
12. Cloud has a spiritual connection with Aeris.
13. Cloud and Aeris are opposites in personality, like Squall and Rinoa, but Aeris can make him smile and laugh and is very flirty

with him like Rinoa is with Squall.

1. When Cloud is impaled by Sephiroth and is taunted by his question "Tell me what you cherish most. Give me the pleasure of taking it away." He immediately sees Aeris, Zack, and lastly Tifa's images. Aeris's image is seen 3 times, while Zack and Tifa's are only seen once.
2. Although Aeris is dead, she still comes to Cloud and tries to get him to forgive himself. It is only through her that Cloud stops feeling guilty. Aeris is the only person that Cloud can truly smile at.
3. Cloud acutally trys on makeup,a dress,and a wig for Aeris.

After Advent Children came out, it just inflamed the entire debate all over again, because to Cleris fans, Cloud seems to abandon Tifa and his home because of guilt over Aeris's death, and because of his geostigma. Additionally, Cloud and Aeris seem to have a more functional relationship than Cloud and Tifa, because although Cloud and Tifa are seen together a lot, they don't seem to understand each other as well as Cloud and Aeris. Also significant is the argument made by these fans that Tifa can't make Cloud happy because he never smiles at her, yet he smiles longingly after Aeris' spirit. In the game, Aeris even makes Cloud laugh.

Reasons People Believe in Cloti

1. Cloud had a crush on Tifa years before he met Aeris.
2. Before Cloud left Nibelheim, he promised Tifa that he would always be there to protect her when she was in danger.
3. Cloud left Nibelheim to be able to protect Tifa, and demonstrate that he was worthy of her.
4. Tifa was the one who helped Cloud regain his true memories.
5. Tifa stayed and helped Cloud when he was hospitalized in Mideel.
6. The doctor in Mideel gave them his blessing.
7. Tifa and Cloud are shown together often in "Advent Children".
8. In the short story "On the Way to a Smile: Case of Tifa" Cloud smiles and says he believes he can start a new life unlike the last time he tried, because Tifa is there with him.
9. Some believe that Aeris was killed simply to make Tifa Cloud's love.
10. Tifa is considered to be Cloud's emotional support in "Advent Children."
11. There is a scene in "Advent Children" where Cloud and Tifa are both unconscious lying in a bed of flowers, the music and atmosphere depict it as a love scene.
12. In Last Order, Cloud and Tifa share a (retconned) moment where Tifa acknowledges Cloud's fulfillment of their promise while Cloud smiles at her.
13. At the end of the game, Tifa and Cloud's themes mix together.
14. Tifa's only storyline seems to revolve around her relationship with Cloud, while Aeris has a story that veers from Cloud.
15. Aeris is dead.
16. Cloud, Tifa, Denzel, and Marlene are shown to be a "family" as indicated by the photo at the beginning of "Advent Children."
17. Reminiscence, the review of Final Fantasy VII, plus short story, indicates Cloud returning to Tifa's bar for a party, while it also shows many of what are considered the 'Cloti' scenes of the game.
18. In the Advent Children Ultimania, the creator states that Tifa is "...like a mother (母のような), sweetheart and fighter..." using the word koibito（恋人）to describe sweetheart, a word exclusively used as lover. Many Cloti fans believe this is evidence that at some point, Cloud and Tifa were lovers.
19. Zack was Aeris' first love.
20. Cloud's broken mind molds memories of Zack's stories into his own memory, thus making it quite probable that Aeris simply thought Cloud was cute, and Cloud had a confused notion that she was his girlfriend (or should be), thus making the relationship an "accident", whereas Tifa is the one to pull Cloud OUT of the mess.

There is a scene between Cloud and Tifa at the end of disc 2 where the party splits up before the final showdown with Sephiroth, to take time out and reflect on each party member's own reasons for fighting. Tifa and Cloud, not having any remaining family or material ties to "go home" to, are left moping alone together under an anchored Highwind for a night. They talk for a while and after a time Tifa remarks to Cloud that "Words aren't the only thing that tell people what you're thinking." The screen fades to black, and they wake up the next morning leaning against each other.