The gospel tells us of two aspects of the Kingdom of God. The first is about growing. In both the parables used, the seed grows and the second is that the tree is meant for the benefit of others.
As we grow in spiritual life through our spiritual exercises and bear the spiritual fruits, we do have to bear in mind that it is for the good of others. This is when the Kingdom of God is present in our midst as Jesus said, the kingdom of God is in your midst (Lk 17:21). The mustard tree in the parable is not just for its own good but for the benefit of others. In our striving to be disciples close to Jesus, it must also be a journey of being caring and loving towards our neighbours.
Can we be tired of growing? If we know who it is we are getting close to through our spiritual growth, we could not grudge continuous growth.