Titus has probably known Lexa for most of her life, and has been with her through all her ups and downs as she continued to grow. Now imagine a young Lexa insecure about her abilities to protect her people, and imagine a younger Titus standing by her side and assuring her that she was special, that she was chosen by the Commander’s spirit for a reason. That she was going to be the one to change the world. Now imagine as young Lexa listening to every word he said with wide eyes and a open heart. She would never forget his words because they would be always in her mind as she grew into the role of the Commander. She believed in his words just as much as he believed in her. 

Now imagine a growing Lexa with Titus still glued to her side, as he continued to stand by and watch this proud Commander do just what he thought she would - and he is proud of her, and he cares for her. He is going to protect his Heda not just because she was chosen, but because she is something dear to him. He has watched her grow into this amazing person who has done more than anyone has ever done before. He is like a proud father, watching his child grow stronger and stronger. 

Honestly, he could be her most loyal friend.