She may want you to be more assertive whenever she is sitting next to you in terms of starting and maintaining a compelling conversation.
If you rarely instigate conversation, you essentially make her feel that she has to be the aggressor when it comes to interaction.
This may be why she picks and chooses the occasions that she will talk to you.
Your passive behavior may also be why she feels emboldened to tease and mock you almost every time that the both of you talk.
If you don’t take charge of the moments that you are sitting next to her, she will feel empowered to do so in whatever manner she chooses.