The first dasha that one experiences in life has a signficant impact on one's life in general. Just by looking at the first dasha lords placement, aspects, yogas, rulership etc. in the chart you can learn a lot about a person. The first dasha is determined by the nakshatra (lunar constellation) that the Moon is placed in at birth. The Moon is the mind (manas), and our early childhood conditioning has a great impact on our mind throughout our life. It's our formative years and leaves a powerful imprint. The first dasha is the same way.
The following chart is of a woman who started her life in her Saturn dasha up until age 12. This is because the Moon is in Pushya, the nakshatra ruled by Saturn. Notice how Saturn is in 8th house and debilitated and unaspected by benefics. It was in this dasha that she experienced several traumatic events that have effected her throughtout her life. She's now in her Venus dasha and because of Venus's placement you might think that married life would be easy for her. Venus is certainly strongly placed in the 7th house as the 9th lord exalted in Pisces and aspected by Jupiter. However, she has had a difficult time in romantic relationships due to the impact of the trauma experienced in her Saturn dasha. Rahu with Venus certainly can suggest some unusual or difficult circumstances in relationship, but the core stems from early childhood experiences the can be gleaned from the first dasha.