Romans in Britain
Romans are dishonest and cruel.
We don't want their foreign gods
Venus and Mars sculpture group reworked to portray an Imperial couple (created 120–140 AD, reworked 170–175)

Boudica's husband Prasutagus was an ally of Rome. When he died, he left his kingdom jointly to his daughters and to the Roman emperor in his will.
However his will was ignored and the kingdom was annexed and his property taken. According to Tacitus, Boudica was flogged and her daughters raped
Lands were confiscated for the benefits of the settlers. A temple to Claudiaus (god emperor) was erected. This temple was seen in British eyes as the "arx aeternae dominationis" ("stronghold of everlasting domination") according to Tacitus
 
Roman exploitation
The Romans have brought their own method of taxation. Each tribe have to pay a levy to a central government based on the yield of crops and consumable items they produced.

The system is prone to abuse: the tax collectors are greedy and dishonest and they always pressure us more than they do.

Besides, the benefits from Roman invasion are not shared equally. For the majority of us, Roman Britain remains a militarised and exploited territory though there are golden opportunities for select individuals amongst the British elite and assorted immigrants. These are the exceptions, participating in government and enjoying a certain level of influence and wealth.
 
FIGHT !
The importance of warfare and the traditions surrounding war is a common pattern throughout Celtic societies and cultures.

Warfare is interwoven into Celtic social structures, art, religion, and lifestyle. Warrior status and prestige is paramount. Our leaders are strong and successful warriors and if we want glory, we need to fight.

War is also the occasion of plunder.
Boudica and her men pillaged as many spoils as they could find in the ruins of Camulodunum before they moved on towards the merchant town of Londinium.
If we join the fight, we might get rich.
 
The Iceni have already won a great battle.
Boudica speaking to the Britons as depicted in 1860

The army of Boudica is at least 10 000 warriors strong. Boudica has led her army towards their first target – the colonia of Camulodunum. (modern Colchester),

They took the city destroyed everything in sight. Dozens of buildings were set on fire and thousands of people were slaughtered in the streets. Boudica did not intend on taking prisoners, nor did her warriors, so any Roman caught by the armed mob was killed.

Our army is strong and merciless and we will defeat any roman legion that may oppose us.