Believers are commanded to celebrate (Ps 68:3, CEB; Phil 4:4, KNT). Indeed, we are to continuously celebrate (1 Thess 2:5, KNT). We have so very much for which we should be enormously grateful!
In Zeph 3:17, it says that God so celebrates having a loving relationship with us that He dances over us (JB), shouts for joy (NJB), and sings loudly (RSV). God says He will celebrate a certain victory in Ps 60:6 (Voice). The Lord especially celebrated His victory over Satan on the cross (Col 2:15, KNT).
What kind of things are we to celebrate? Our friendship with God is worth celebrating (Jn 15:14, Voice)! Likewise, we ought to celebrate that we will see the Lord (Jn 8:56, KNT). That we have hope is a reason to rejoice (Rom 12:12, KNT). Our calling (Rom 11:12, KNT) and the fact that others care about us are causes for celebration (Phil 4:10, KNT). In addition, we can express joy that God’s will is victorious (NIV Thematic Bible, p 2033-2034). Naturally we can be happy on occasions like a wedding (Jn 2) or bringing the Ark to Jerusalem (2 Sam 6). Besides all this, it is incredibly appropriate for us to rejoice that we will be going to heaven (Lk 15:7, 10).
What benefit is there to celebrating? First, we celebrate that God is shaping our character through hardships (Rom 5:3-4, Voice). Second, we are joyful because our sufferings can benefit others (Col 1:24, KNT). Third, being festive helps us see God (Jn 8:56, NASB alt). Fourth, celebrating makes each day like a feast (Prov 15:15, CCB). Fifth, it can free us from anxiety (Phil 4:6). Other benefits listed by Foster include laughter, appreciation for life, ability to enjoy life, healing, and not taking ourselves too seriously (Foster, Celebration of Discipline, p 197). It is worth it to celebrate and delight ourselves in the Lord (Ps 37:4).
Instead of being timid, learn to be appropriately bold.