There are three reasons why we should tithe.
Tithing reflects our heart.
Bring all the tithes into the storehouse, that there may be food in My house, and try Me now in this,” says the Lord of hosts, “If I will not open for you the windows of heaven and pour out for you such blessing that there will not be room enough to receive it.
Tithing is biblical.
Then Melchizedek king of Salem brought out bread and wine; he was the priest of God Most High. And he blessed him and said: “Blessed be Abram of God Most High, Possessor of heaven and earth; and blessed be God Most High, who has delivered your enemies into your hand.” And he gave him a tithe of all.
And this stone which I have set as a pillar shall be God’s house, and of all that you give me I will surely give a tenth to you.
And all the tithe of the land, whether of the seed of the land or of the fruit of the tree, is the Lord’s. It is holy to the Lord.
And it shall be, when you come into the land which the Lord your God is giving you as an inheritance, and you possess it and dwell in it, that you shall take some of the first of all the produce of the ground, which you shall bring from your land that the Lord your God is giving you, and put it in a basket and go to the place where the Lord your God chooses to make His name abide.
Tithing is a benefit.
2 Chronicles 31:4-10
Moreover he commanded the people who dwelt in Jerusalem to contribute support for the priests and the Levites, that they might devote themselves to the Law of the Lord.
As soon as the commandment was circulated, the children of Israel brought in abundance the first fruits of grain and wine, oil and honey, and of all the produce of the field; and they brought in abundantly the tithe of everything. And the children of Israel and Judah, who dwelt in the cities of Judah, brought the tithe of oxen and sheep; also the tithe of holy things which were consecrated to the Lord their God they laid in heaps.
In the third month they began laying them in heaps, and they finished in the seventh month. And when Hezekiah and the leaders came and saw the heaps, they blessed the Lord and His people Israel. Then Hezekiah questioned the priests and the Levites concerning the heaps. And Azariah the chief priest, from the house of Zadok, answered him and said, “Since the people began to bring the offerings into the house of the Lord, we have had enough to eat and have plenty left, for the Lord has blessed His people; and what is left is this great abundance.