"Martha, Martha, you are worried and upset about many things ..." (Luke 10:41-42)

"Martha, Martha, you are worried and upset about many things, but few things are needed—or indeed only one. Mary has chosen what is better, and it will not be taken away from her." (Luke 10:41-42)

Why is Jesus telling Martha this?

Jesus is clarifying an important lesson regarding our consciousness. What does it mean? Here is the context of Jesus' statement to Martha:
As Jesus and his disciples were on their way, he came to a village where a woman named Martha opened her home to him. She had a sister called Mary, who sat at the Lord's feet listening to what he said. But Martha was distracted by all the preparations that had to be made. She came to him and asked, "Lord, don't you care that my sister has left me to do the work by myself? Tell her to help me!" (Luke 10:38-40)
So we find that Martha is hosting Jesus and some students including Martha's sister Mary. Martha was busy with the arrangements while Mary was focused on what Jesus was teaching. Martha became upset that Mary wasn't helping her. She then interrupted Jesus to voice her concern. So Jesus was responding to this.

What Jesus is speaking of relates to our consciousness and focus in life. Where was Martha's focus and where was Mary's focus? Mary's focus was upon hearing from her teacher, Jesus - and thus learning about God and how to learn to love God.

Martha's focus was on herself. She was upset that Mary wasn't helping her.

While some might say that Martha was doing an important thing - as someone had to make the preparations to host Jesus and his students at her house - Jesus is acknowledging this necessity but stated that "few things are needed" - this means that Martha didn't need to make extravagant preparations. She also did not need to become upset and worried about them.

Then Jesus focuses on the prime reason for his being there in the first place: He is teaching his students about the Supreme Being. He is helping re-introduce them to God and helping them re-develop their relationship with God.

What is the 'better' thing that Mary has chosen?

Jesus also indicates Mary has chosen "what is better," and this better thing "will not be taken away from her." What does this better thing?

Each of us is occupying a physical body for a temporary period of time. This means that all of this stuff around us - our house, our job, our money, our reputation, and all the other trappings of this world - will dissolve when our physical body dies.

This means that this is all temporary. The physical body is temporary and all the trappings of the body - including family, job, house and so on - are all temporary. Why should we get so worked up about all this temporary stuff? Why get so worked up about our reputation and our car and so many other things that will simply dissolve in a few years?

But our relationship with the Supreme Being is eternal. It continues after the death of the physical body. This is "what is better" according to Jesus.

You see, beneath these physical bodies we are each an eternal spirit-person. And this eternal spirit-person has a spiritual form. This spirit-person also has a particular relationship with the Supreme Being.

We are here to achieve our eternal relationship with the Supreme Being. Our ultimate success is reclaiming our lost relationship with God. This is the purpose of our learning experiences here in the physical world, a place that provides us with choices.

Jesus is teaching us how we can return home to God. How we can re-develop our loving relationship with the Supreme Being. This is why Jesus was trying to encourage Martha to focus on his teachings and not on the mundane aspects of the physical world. This is also why Jesus' most important instruction was:
“‘Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment.” (Matt. 22:37-38)