Well, that was unexpected

Love and Peace

When they were children, they had been close. Tobirama had clung to his eldest brother's side from the moment he awoke until the moment he fell asleep, lulled into rest by Hashirama's hands running through his hair and listening to Hashirama talk of his dreams for the future, Itama and Kawarama snoring softly from the two beds next to his. He had admired the strength of his brother, his will, and his goals to protect not only himself and his brothers, but the whole world. Even from a young age, Hashi dreamed of peace. After their brothers had died, it only drove his wish even further. So Tobirama had followed him, watching him, learning from him. He had idolized his brother and followed him everywhere.
So of course he had noticed when Hashirama started to meet with Madara.
His brother had no sense of modesty. He had always taken every opportunity to show off his skills and knowledge, which never failed to leave Tobirama with stars in his eyes. However, Madara wasn't so easily impressed. Hashirama had thought he was teaching Madara something when he had skipped that first stone, thought that by showing off he was both showcasing his talent and educating the boy. Madara, however, wasn't impressed. Quite the opposite really. He had gotten angry at the appearance of some nuisance who thought he knew better than Madara did. He didn't need his help. It was just skipping stones for god's sake. And he wasn't afraid to let Hashirama know exactly how he felt.
When Madara's anger flared, so did his chakra, which reached deep into the line of trees where Tobirama had been hiding. He had only come to observe his brother, to learn his skills and to admire from afar, but he was instead introduced to his new favorite thing. Madara's chakra had been absolute heaven to the boy. It had so quickly taken over all of his senses and clouded his mind with its sudden high that Tobirama had fallen from his perch in the tree and into the bushes beneath him filled with thorns. Even while being scraped up and forced to bleed, his mind took quite a while to register that he was in pain at all. And when he did wake up, he had pulled himself up, ignored his bloodied limbs (neither father nor Hashirama noticed the scrapes later that day), and moved almost to the edge of the woods where he could see the source of that little slice of heaven he'd sensed. And Madara hadn't disappointed.
After that, Tobirama followed his brother into the woods everyday. He spent less time watching Hashi and more savoring the glimpses he got of that regal face and waiting for those bursts of amazing chakra. Madara's chakra was comforting and warm, but it also carried such raw and pure emotion that it left Tobirama near the brink of tears. When Madara felt, it was so much different than Hashirama. Hashi's emotions had always been so quick to change and were hardly ever genuine, but Madara was always so true. Tobirama wished to one day be everything Madara embodied.
The first time he felt another chakra nearby, he had tried to ignore it. It had come from across the river, and Tobirama had at first dismissed it. It was only one chakra, and not particularly strong which indicated that it wasn't some sort of ambush. Just as he was about to close his eyes again to relish Madara's chakra and to listen to his sweet voice bicker with Hashirama, he had realized how close it was to Madara's. They were so close, but this one was tainted with something wicked rather than the kindness and purity he felt in the other boy's. He had moved himself closer to the edge of the woods to leap into action in the case of attack, but it never came. The chakra stayed still, and waited until Madara ventured away towards home.
That chakra reappeared day after day, always coming in right after Madara and leaving right as Madara said his goodbyes to Hashirama. Tobirama had long since assumed that it was someone from Madara's clan coming to watch, and he was proven right one day when he felt that chakra speeding off towards which it came only an hour after arriving. He reluctantly left his perch in his favorite tree (it was the optimal spot for viewing and sensing the boy's chakra), and followed it. When he arrived at the Uchiha compound, he was stunned. He had never expected Madara to be of that clan, not when he was so kind and pure and so delicious (no Tobi bad Tobi bad bad bad). And when that mysterious chakra was fondly called Izuna by the clan head, he instantly knew what was happening. Madara was not only an Uchiha, he was the Uchiha. The son of the clan head, and quite possibly his brother's demise. And so he fled.
The Uchiha were planning to kill his brother the next day. He needed to warn father.