The TARDIS loved when her thief laughed. She loved to hear the joy in his voice, to sense the smile on his face. When The Doctor was happy, his voice filled the corridors, and he stroked her console affectionately. When things were good, he ate the fish fingers and custard she would make for him. They could drift through space happily.
But at night, he was sad. She could sense the anguish and pain he held inside. He would toss and turn at night and cry out occasionally. Even with her, his magic blue box, he still needed companionship, someone to speak to, and hear respond back.
The TARDIS was incapable of speaking back.
All she could do was put a soothingly melody in his head at night and hope her thief slipped off to sleep. Sometimes it worked, a lot of the times it didn't. It pained her and the hum of her power would change to a sad melody. Come morning her Time Lord would stroke her console and talk to her, assure her things were fine. She knew better.
What her thief needed was a stranger, preferably a human. The blue box knew what her Time Lord needed all too well. While she never took him where he wanted to go, she always knew where he needed to go, and that is why she took him to Earth. What The Doctor needed most was love, a home, a planet to call home.
Things never always went as planned. These days her Doctor refused to go outside. He neither laughed, nor sang to her in Gallifreyan. He did not eat fish fingers and custard. His mind was not filled with happy times, nor did he dream of past companions.
Her thief was changing, no longer the happy Doctor who fixed people. She did not like that he only wanted to be inside of her safe walls. There were no stories to tell, no conversations had. Most of all, he did not pilot her to see the stars.
This was not her thief, but she was a simple machine. The TARDIS had a heart, she could not force him to go and find what he needed. All she could do was hope someone would stumble up on them and make her thief see what he is missing.
When that happens she will fix The Doctor and his new companion fish fingers and custard. He will laugh and fill the halls with chatter. When alone, her time Lord will call her Sexy and stroke her console affectionately. They will talk of adventures through their symbiotic link. His joy will radiate towards her, and she will hum contentedly. They will sail off to see the stars.
But inevitably the cycle will repeat itself. She'll worry about how The Doctor will react and try to keep his restless mind occupied. Thus is their relationship as she faithfully serves by his side. A stolen blue box and the last Time Lord, traveling the stars together,