First Communion is often a very joyful time for families, because the person receives the Body, Blood, Soul and Divinity of Jesus for the first time. Particularly in the western Church, receiving Communion only happens when the child understands what he or she is receiving. Thus, it signifies a level of maturity and understanding in the child. It is also very joyous because they are receiving such an awesome gift from God. They have now been afforded the privilege of receiving Jesus into themselves in a very unique and personal way and of receiving the multitudinous graces which go along with that.
It is the beginning of many graces
This is a very important step in the spiritual development of a person. It opens up a lifetime of opportunity to go to Jesus in the Blessed Sacrament and receive Him sacramentally into oneself for a greater union with Him. Hopefully, this is the beginning of a lifetime of a close relationship with Christ and the opportunity to grow ever closer to Him. Whenever there are difficulties in life, this is one place that a person can go to receive all the graces necessary to make it through that rough patch.
A First Communion should be seen not as a one-time celebration, but rather as the opening of a wellspring of grace, to be dipped into frequently. Since at the time of a first communion the child is often still relatively young, at only seven or eight, it is so important to celebrate this occasion worthily in order to reinforce in their minds that this is a special gift and that it should not be taken lightly or for granted. Hopefully they will continue to avail themselves of the sacrament throughout their lives.