Ares was the God of War who loved to stir up trouble all over Greece (it was in his job description after all).  He nurtured promising warriors into successful warlords and discarded them when their job was done or when they became bothersome.  Xena was one such warrior, and she became a great warlord with his help.  I believe that they had a bit of a romantic fling while she worked for him, but that's just based on the way he acted towards her in the show.

Xena was a simple village girl from the town of Amphipolis, but she followed the path of war when her village was attacked by a warlord.  She became a great warlord herself and rode around Greece with her army, leaving a path of destruction in her wake.  She acquired quite a reputation for herself, but finally realized the error in her ways with Hercules' help.  Though she began to work for the side of good, it took awhile to untarnish her reputation.  She never fully rid herself of her dark past, but people came to trust her to help them.

Ares always wanted Xena to come back to work for him and the dark side, but she repeatedly refused.  When she and Gabrielle were "killed", Ares buried them in an ice tomb and mourned her death.  Xena and Gabrielle came back about 20 years later and Ares realized even more how much Xena meant to him.  He never lost the love that he had for her, if anything it increased during their separation, but she didn't want anything to do with him.  He tried to be kind to her, but she always thought that he just wanted to get something from her.  She was right (he did want her to give him a child), but it was never clear if his motives were malicious or because he loved her and wanted part of her in his life.

While Xena and Gabrielle were frozen, Ares made Xena's daughter Eve into a great warrior.  He had a bit of a relationship with Eve, but once he saw Xena again, all of his old feelings for her rushed back to the forefront.  He always loved her and always will.  He even gave up all of his powers and his immortality to save Eve and Gabrielle's lives.  He could have let the two die - it would have even saved his life as a god on Olympus - but he had to do it for Xena.  She cared about the two of them, and so Ares proved his love for Xena in the most permanent and drastic way he could.  Even though Xena didn't seem to care for Ares in the same way that he cared for her, she at least realized how deep his feelings actually were.  Through loving Xena, Ares was able to change his ways as she did.

When Ares became a mortal, he found it hard to adjust to life as one.  He didn't want all of his armies to know that he had lost his powers, but the Furies and Xena had other plans for him.  The Furies took over Ares' head and made him appear to be going crazy.  Xena came up with a plan to release him from the Furies grasp that involved her faking her own death.  Ares's love for Xena allowed him to break free of the Furies and save her life.  Xena helped Ares because even though she didn't want to, she did care about him and didn't want to see him suffer.  And though she did have feelings for him, Xena couldn't allow herself to give into a relationship with someone who brought her so much pain and trouble throughout her life.  Ares was a changed man because of Xena though, so had they been given more time, perhaps they could have come together.

Xena and Ares do not have a future together based on the end of the show.  Ares gave up his godhood to save her daughter and Gabrielle, and Xena died.  It was pretty disappointing that their relationship couldn't have worked out, but more fans supported Xena's relationship with Gabrielle.  The way the show went, he would live out the rest of his life as a mortal without Xena by his side.  He may have a few flings here and there, but I don't see him making a commitment to anyone.  His feelings for Xena seemed to run too deep for him to find someone else to fill the void that her death undoubtedly left within his heart.

This scene is from the episode "Livia".  Ares found out that Xena was alive earlier that day when she broke up his kiss with his newest warrior Livia.  Xena came back to talk with him later.

Ares: "I was kinda hoping you'd be back.  I gotta know what happened.  I thought you were dead."

Xena: "That's a nasty rumor."

Ares: "No, no.  See, I mourned you.  For years I mourned you."

Xena: "I can see you looked positively grief-stricken when I saw you today at the arena."

Ares: "What? That rough and tumble with Livia?  You're not jealous."

Xena: "Of a Roman wannabe?  Hardly."

Ares: "Well you're right, she's not you. Because if I thought for one moment that you being here meant we had a second chance."

Xena: "Yes?"

Ares: "I'd drop her like a nasty habit (they kiss)."

~~~

This scene is from the episode "Motherhood".  Xena went outside the house they were staying in because she heard something, and she found Ares there.  She drew her sword and chakrum, but stopped short of killing him.

Ares: "You're not gonna kill me are you?"

Xena: "Why ever not?"

Ares: "Because I love you. Yeah, I love you."

~~~

This scene is from the same episode as the one above.  Ares gave up his godhood to save Eve and Gabrielle, thereby giving Xena the power to kill gods once again.  This scene is between Athena and Ares, right after Xena stabbed her.

Athena: "You healed them without my blessing?  That's impossible!"

Ares: "I gave up my immortality to save them."

Athena (in disbelief): "Why?"

Ares: "I'm sorry, but I've got a thing for her."

~~~

This scene is from the end of "Coming Home".  Ares and Xena are having a chat after she helped get the Furies out of his head.

Xena: "Hey."

Ares: "Mortality stinks.  Not to mention hurts."

Xena: "Yeah, it does sometimes."

Ares: "You have got a wicked right-jab."

Xena: "Sorry about that." (she smiles)

Ares: "It's not the end of the world.  Who knows, maybe as a mortal I can experience something I never could as a god. "

Xena: "Like what?"

Ares: "Like you."

Xena: "You never give up, do you?" (they kiss)

Ares: "Ow (they laugh).  I guess I'm finally getting to you."

Xena: "You always got to me, but you were bad for me Ares, you still are." (she walks away)

Ares: "Xena!  Is there even a chance we could be together one day?  I don't know, maybe one in a thousand?"

Xena: "More like one in a billion."

Ares: "So you're saying there is a chance?" (she chuckles and smiles at him)