For where your treasure is, there will your heart be also (Matthew 6:19-21).
Everyone in the modern Church in the west acknowledges that Jesus told us to love one another (John 13:34-35; 15:12). Everyone is of course, very happy to hear that because of their own selfishness in expecting others to love them.
However, the same God who told us to love one another and love one's enemies (Matthew 5:43-48; Luke 6:27-36), also said that no one can serve both God and mammon (Matthew 6:24) and to give up one's whole self to follow Him (Matthew 16:24; Mark 8:34; Luke 9:23), which so many in the modern Church in the west find so hard to accept.
Few churches preach on this topic, and even where it is preached about, these teachings are softened. To soften any teaching of the Bible is itself to deny the power, and therefore the teaching for what it is, for the teaching lies in the power, the power of convict, and yes, the power to offend. The offensive power of not only the Gospel of Jesus Christ (Romans 1:16), but any teaching which the sinner or even the saved is seeking to rebel against owing to succumbing to the flesh, is the power of the Word of God.
It is not something the Christian should be afraid of in bring the Gospel of Jesus Christ to the world, or something a pastor or preacher should be afraid of in preaching hard teachings. Rather, one should be all the more encouraged by the offensive power of the Word of God, because it is this very offensiveness to the rebellious, sinful, wicked, insubordinate, evil flesh that God is all the more gloried.
Storing up treasures on earth is simply to pursue more earthly material wealth, riches and comfort. To even think about seeking basic earthly comfort, a life where there is no hardship is itself to store up such earthly treasure. Earthly treasures does not only include money, riches and wealth. It includes absolutely anything that will give one a more comfortable life. It can include food, water, clothing, and place to live.
You may say that food, water, clothing and a place to live are legitimate earthly needs, such that seeking these things could not be what Matthew 6:19-21 refers to. However, this argument simply is one which has a low view of that teaching, in seeking to reduce what it really means. While they are legitimate earthly needs, this does not mean all of a sudden that one can set one's mind on them in pursuing them for oneself. Such a belief would be of the respect of persons, full of hypocrisy in selecting which part of God's Word one wants to believe in, while rejecting the part of His teachings that one does not want to obey.
Jesus said in Matthew 6:31-32 that the pagans, or unsaved people seek after their things:
Therefore take no thought, saying, What shall we eat? or, What shall we drink? or, Wherewithal shall we be clothed?
(For after all these things do the Gentiles seek:) for your heavenly Father knoweth that ye have need of all these things.
Before that, He was explaining what it means to take thought of such things:
No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.
Therefore I say unto you, Take no thought for your life, what ye shall eat, or what ye shall drink; nor yet for your body, what ye shall put on. Is not the life more than meat, and the body than raiment?
Behold the fowls of the air: for they sow not, neither do they reap, nor gather into barns; yet your heavenly Father feedeth them. Are ye not much better than they?
Which of you by taking thought can add one cubit unto his stature?
And why take ye thought for raiment? Consider the lilies of the field, how they grow; they toil not, neither do they spin:
And yet I say unto you, That even Solomon in all his glory was not arrayed like one of these.
Wherefore, if God so clothe the grass of the field, which to day is, and to morrow is cast into the oven, shall he not much more clothe you, O ye of little faith?
In Matthew 6:24-25, He said that because no one can serve both God and mammon, do not think about what you shall eat, drink, or wear, as indicated by the use of the word 'therefore' as a conjunctive in verse 25, which is connects to verse 24.
That is simply what it means. It is very simple to understand.
But what makes it so "hard" to understand is that many in the modern Church in the west don't want to obey it, and so therefore they seek to qualify or soften this teaching. It is that simple.